Interrogator and Interrogation System Employing the Same

ABSTRACT

A control and processing system for use with an interrogator and an interrogation system employing the same. In one embodiment, the control and processing system includes a correlation subsystem having a correlator that correlates a reference code with a reply code from a radio frequency identification (RFID) tag and provides a correlation signal therefrom. The control and processing system also includes a decision subsystem that verifies a presence of the RFID tag as a function of the correlation signal.

This is a Continuation of application Ser. No. 16/853,673, entitled“Interrogator and Interrogation System Employing the Same,” filed onApr. 20, 2020, now U.S. Pat. No. 11,205,058, which is a Continuation ofapplication Ser. No. 16/252,652, entitled “Interrogator andInterrogation System Employing the Same,” filed on Jan. 20, 2019, nowU.S. Pat. No. 10,628,645, which is a continuation of Application No.15/647,131, entitled “Interrogator and Interrogation System Employingthe Same,” filed on Jul. 11, 2017, now U.S. Pat. No. 10,185,853, whichis a Continuation of application Ser. No. 14/611,425, entitled“Interrogator and Interrogation System Employing the Same,” filed onFeb. 2, 2015, now U.S. Pat. No. 9,710,683, which is a Continuation ofapplication Ser. No. 11/071,652, entitled “Interrogator andInterrogation System Employing the Same,” filed on Mar. 3, 2005, nowU.S. Pat. No. 8,948,279, which claims the benefit of U.S. ProvisionalApplication No. 60/549,853, entitled “An Interrogator and InterrogationSystem Employing the Same,” filed on Mar. 3, 2004, which areincorporated herein by reference.

TECHNICAL FIELD

The present invention is directed, in general, to communication systemsand, more specifically, to an interrogator, method of discerning radiofrequency identification (RFID) objects, and an interrogation systememploying the same.

BACKGROUND

Asset tracking for the purposes of inventory control or the like isemployed in a multitude of industry sectors such as in the foodindustry, apparel markets and any number of manufacturing sectors, toname a few. In many instances, a bar coded tag or radio frequencyidentification (RFID) tag is affixed to the asset and a readerinterrogates the item to read the tag and ultimately to account for theasset being tracked. Although not readily adopted, an analogous systemmay be employed in a medical environment to track equipment such as anElectrocardiogram (EKG) machine or other modular patient monitoringequipment.

Of particular note is a surgical environment in which for preparationfor surgery a previously sterilized instrument kit of surgicalinstruments and disposable items (collectively referred to as surgicalitems) is brought into a surgical suite. The instrument kit contains anassortment of surgical items including hemostats, clamps, forceps,scissors, sponges, and the like, based on the type of surgery to beperformed. Typically, a scrub nurse removes the surgical items from thekit and arranges them on a back table located behind the operatingtable. The surgical items are organized in rows on rolled toweling forease of access and handling by a surgeon and supporting team. During thecourse of a surgical procedure, the surgical items are often positionedon a “Mayo” stand proximate the operating table, while the unusedsurgical items remain on the back table. During the course of and at theconclusion of the surgery, all of the surgical items must be carefullycounted to, among other things, avoid leaving any surgical items in apatient.

In view of the consequences, surgical items are typically counted atleast three times during the course of a surgical procedure. The firstcount is performed prior to the start of the procedure; the second countis performed prior to a closure of the patient; the third count isperformed at the conclusion of the procedure. In many instances such aswhen more than one surgical team is assigned to a procedure, many morecounts of the surgical items, often involving different personnel (e.g.,a circulating nurse and a scrub nurse), are performed. As a matter offact, the Association of PeriOperative Registered Nurses (AORN)advocates four counts of the surgical items as part of its recommendedpractices for surgical procedures. Additionally, to keep track of thecounts of the surgical items, rudimentary systems such as visual recordsscribbled on whiteboards or other more progressive computer tallyingsystems to designate the count of the surgical items are often employed.

In common practice, access to and from an operating room in the surgicalsuite is restricted during the counting process thereby resulting in adetention of valuable professional personnel. A discrepancy in the countmust be resolved by additional counts, physical examination of thepatient or x-ray examination, if necessary. Although it is unusual for adiscrepancy in the count to result from a surgical item remaining in thepatient, counting and recounting occurs in every surgical procedure andthe repercussions associated with the loss of a surgical item is ofgrave concern to a medical facility and the medical professionals.

Thus, the multiple manual counting of surgical items is time consuming,ties up key professional personnel, contributes to surgical suite downtime, distracts personnel from the surgical procedure, lengthens thetime the patient is exposed to anesthesia leading to an increase inmortality and morbidity risk, is generally distasteful to all involved,and still results in errors wherein materials are left in the patient.It should be quite understandable that the average cost overruns of suchdelays associated with the personnel, capital equipment and the surgicalsuite itself can run into the tens of thousands of dollars perprocedure. On an annual basis, the loss of productivity associated withthe surgical suite is quite sizeable and should be addressed to bolsterthe bottom line of a medical facility.

Even with the degree of caution cited above, the problem associated withthe loss of surgical items, especially surgical items retained withinpatients, is a serious one and has a significant influence on the costsof malpractice insurance. As a matter of fact, retained foreign bodieswithin a patient is one of the most prevalent categories of malpracticeclaims and the most common retained foreign body is a sponge. Inaccordance therewith, there is a diagnosis known as “gossypiboma”(wherein gossypium is Latin for cotton and boma is Swahili for place ofconcealment) for the retention of a sponge-like foreign body in apatient. The medical literature is scattered with reports ofpresentations of retained sponges found days, months, or even yearsafter a surgical procedure.

The sponge is typically made of gauze-like material with dimensionsoften covering a four-inch square or a two-inch by four-inch rectangle.At one time sponges were commonly made of cotton, but now a number offilament materials are used. Occasionally, a filament of radiopaquematerial [e.g., barium sulfate (BaSO₄)] is woven into the surgicalsponge, or a tab of that material is attached to the surgical sponge.The filament or tab is provided to produce a distinct signature on anx-ray machine for the purpose of determining if a sponge is present inthe patient. While this is generally effective, even these filaments ortabs are not 100% effective in aiding the location of the sponges.Different researchers report that x-ray methods to supplement manualcounting are fallible.

Moreover, in cases when a sponge remains in the body for a long time,the radiopaque filament can become difficult to locate and may evenconform to internal structures. Some have suggested that a computerizedtomography (CT) scan can be more effective than an x-ray examinationbecause the CT scans and ultrasonography may detect the reduced densityof a sponge and its characteristic pattern of air or gas bubbles trappedwithin the sponge. Many radiologists have published a number of papersover the years on the problem of finding lost sponges and these aregenerally known in the field of medicine.

As mentioned above, there is a widespread practice in other fields forcounting, tracking and accounting for items and two of the moreprevalent and lowest cost approaches involve various types of bar codingand RFID techniques. As with bar coding, the RFID techniques areprimarily used for automatic data capture and, to date, the technologiesare generally not compatible with the counting of surgical items. Areason for the incompatibility in the medical environment for the barcoding and RFID techniques is a prerequisite to identify items coveredin fluids or waste, and the exigencies associated with the sterilizingof surgical items including a readable tag.

Even in view of the foregoing limitations for the application of RFIDtechniques in the medical environment wherein less than ideal conditionsare prevalent, RFID tags have been compatible with a number of arduousenvironments. In the pharmaceutical industry, for instance, RFID tagshave survived manufacturing processes that require products to besterilized for a period of time over 120 degrees Celsius. Products areautoclaved while mounted on steel racks tagged with an RFID tag suchthat a rack identification (ID) number and time/date stamp can beautomatically collected at the beginning and end of the process as therack travels through the autoclave on a conveyor. The RFID tags can bespecified to withstand more than 1000 hours at temperatures above 120degrees Celsius. This is just one example of how RFID tags can withstandthe arduous environment including the high temperatures associated withan autoclave procedure, whereas a bar code label is unlikely to survivesuch treatment.

While identification tags or labels may be able to survive the difficultconditions associated with medical applications, there is yet anotherchallenge directed to attaching an identification element to a surgicalitem or any small device. The RFID tags are frequently attached todevices by employing mechanical techniques or may be affixed with sewingtechniques. A more common form of attachment of an RFID tag to a deviceis by bonding techniques including encapsulation or adhesion.

While medical device manufacturers have multiple options for bonding,critical disparities between materials may exist in areas such asbiocompatibility, bond strength, curing characteristics, flexibility andgap-filling capabilities. A number of bonding materials are used in theassembly and fabrication of both disposable and reusable medicaldevices, many of which are certified to United States PharmacopoeiaClass VI requirements. These products include epoxies, silicones,ultraviolet curables, cyanoacrylates, and special acrylic polymerformulations.

In many instances, the toughness and versatile properties ofbiocompatible epoxies make them an attractive alternative. Epoxies formstrong and durable bonds, fill gaps effectively and adhere well to mosttypes of substrates. Common uses for medical epoxies include a number ofapplications which require sterilization compatibility such as bondinglenses in endoscopes, attaching plastic tips to tubing in disposablecatheters, coating implantable prosthetic devices, bonding balloons tocatheters for balloon angioplasty, and bonding diamond scalpel bladesfor coronary bypass surgery, to name a few. A wide range of suchmaterials are available and some provide high strength bonds which aretough, water resistant, low in outgassing, and dimensionally stable overa temperature range of up to 600 degrees Fahrenheit. Some epoxies canwithstand repeated sterilization such as autoclaving, radiation,ethylene oxide and cold (e.g., chemical) sterilization methods.

As previously mentioned, familiar applications for RFID techniquesinclude “smart labels” in airline baggage tracking and in many storesfor inventory control and for theft deterrence. In some cases, the smartlabels may combine both RFID and bar coding techniques. The tags mayinclude batteries and typically only function as read only devices or asread/write devices. Less familiar applications for RFID techniquesinclude the inclusion of RFID tags in automobile key fobs as anti-theftdevices, identification badges for employees, and RFID tags incorporatedinto a wrist band as an accurate and secure method of identifying andtracking prison inmates and patrons at entertainment and recreationfacilities. Within the medical field, RFID tags have been proposed fortracking patients and patient files, employee identification badges,identification of blood bags, and process management within thefactories of manufacturers making products for medical practice.

Typically, RFID tags without batteries (i.e., passive devices) aresmaller, lighter and less expensive than those that are active devices.The passive RFID tags are typically maintenance free and can last forlong periods of time. The passive RFID tags are relatively inexpensive,generally as small as an inch in length, and about an eighth of an inchin diameter when encapsulated in hermetic glass cylinders. Recentdevelopments indicate that they will soon be even smaller. The RFID tagscan be encoded with 64 or more bits of data that represent a largenumber of unique ID numbers (e.g., about 18,446,744,073,709,551,616unique ID numbers). Obviously, this number of encoded data provides morethan enough unique codes to identify every item used in a surgicalprocedure or in other environments that may benefit from asset tracking.

An important attribute of RFID interrogation systems is that a number ofRFID tags should be interrogated simultaneously stemming from the signalprocessing associated with the techniques of impressing theidentification information on the carrier signal. A related anddesirable attribute is that there is not typically a minimum separationrequired between the RFID tags. Using an anti-collision algorithm,multiple RFID tags may be readily identifiable and, even at an extremereading range, only minimal separation (e.g., five centimeters or less)to prevent mutual de-tuning is generally necessary. Most otheridentification systems, such as systems employing bar codes, usuallyimpose that each device be interrogated separately. The ability tointerrogate a plurality of closely spaced RFID tags simultaneously isdesirable for applications requiring rapid interrogation of a largenumber of items.

In general, the sector of radio frequency identification is one of thefastest growing areas within the field of automatic identification anddata collection. A reason for the proliferation of RFID systems is thatRFID tags may be affixed to a variety of diverse objects (also referredto as “RFID objects”) and a presence of the RFID tags may be detectedwithout actually physically viewing or contacting the RFID tag. As aresult, multiple applications have been developed for the RFID systemsand more are being developed every day.

The parameters for the applications of the RFID systems vary widely, butcan generally be divided into three significant categories. First, anability to read the RFID tags rapidly. Another category revolves aroundan ability to read a significant number of the RFID tags simultaneously(or nearly simultaneously). A third category stems from an ability toread the RFID tags reliably at increased ranges or under conditionswherein the radio frequency signals have been substantially attenuated.While significant progress has been made in the area of reading multipleRFID tags almost simultaneously (see, for instance, U.S. Pat. No.6,265,962 entitled “Method for Resolving Signal Collisions BetweenMultiple RFID Transponders in a Field,” to Black, et al., issued Jul.24, 2001, which is incorporated herein by reference), there is stillroom for significant improvement in the area of reading the RFID tagsreliably at increased ranges or under conditions when the radiofrequency signals have been substantially attenuated.

Accordingly, what is needed in the art is an interrogator, interrogationsystem and related method to identify and account for all types of itemsregardless of the environment or application that overcomes thedeficiencies of the prior art.

SUMMARY OF THE INVENTION

These and other problems are generally solved or circumvented, andtechnical advantages are generally achieved, by advantageous embodimentsof the present invention which includes an interrogator and aninterrogation system employing the same. In one embodiment, theinterrogator includes a control and processing system having acorrelation subsystem that correlates a reference code with a reply codefrom a radio frequency identification (RFID) tag and provides acorrelation signal therefrom. The control and processing system alsoincludes a decision subsystem that verifies a presence of the RFID tagas a function of the correlation signal.

In another aspect, the present invention provides an interrogatorincluding an RFID sensing subsystem that provides a reply codeassociated with an RFID tag and a control and processing subsystem. Thecontrol and processing subsystem includes a correlation subsystem thatcorrelates a reference code with the reply code and provides acorrelation signal therefrom. The control and processing subsystem alsoincludes a decision subsystem that verifies a presence of the RFID tagas a function of the correlation signal. The interrogator may alsoinclude a metal sensing subsystem that provides a signal having a metalsignature representing a presence of a metal object. The control andprocessing subsystem may be configured to discern a presence of themetal object from the signal.

In yet another aspect, the present invention provides an interrogationsystem including a computer system and a transceiver that transmits andreceives signals for the computer system. The interrogation system alsoincludes an interrogator having an RFID sensing subsystem that providesa reply code associated with an RFID tag of an RFID object. Theinterrogator also includes a control and processing subsystem having acorrelation subsystem that correlates a reference code with the replycode and provides a correlation signal therefrom. The control andprocessing subsystem also includes a decision subsystem that verifies apresence of the RFID tag as a function of the correlation signal. Theinterrogator also includes a communications subsystem that communicateswith the transceiver.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention in order that the detaileddescription of the invention that follows may be better understood.Additional features and advantages of the invention will be describedhereinafter which form the subject of the claims of the invention. Itshould be appreciated by those skilled in the art that the conceptionand specific embodiment disclosed may be readily utilized as a basis formodifying or designing other structures or processes for carrying outthe same purposes of the present invention. It should also be realizedby those skilled in the art that such equivalent constructions do notdepart from the spirit and scope of the invention as set forth in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, reference isnow made to the following descriptions taken in conjunction with theaccompanying drawings, in which:

FIG. 1 illustrates a diagram of an embodiment of a communication systememployable in a medical environment constructed in accordance with theprinciples of the present invention,

FIG. 2 illustrates a pictorial diagram of an embodiment of aninterrogation system employable within an operating room of a medicalfacility and constructed in accordance with the principles of thepresent invention,

FIG. 3 illustrates a pictorial diagram of an embodiment of aninterrogation system employable within an operating room of a medicalfacility and constructed in accordance with the principles of thepresent invention,

FIG. 4 illustrates a pictorial diagram of an embodiment of aninterrogator constructed in accordance with the principles of thepresent invention,

FIG. 5 illustrates a system diagram of an embodiment of an interrogatorconstructed in accordance with the principles of the present invention,

FIG. 6 illustrates a block diagram of another embodiment of aninterrogator constructed in accordance with the principles of thepresent invention,

FIG. 7 illustrates a system diagram of an alternative embodiment of aninterrogator constructed in accordance with the principles of thepresent invention,

FIG. 8 illustrates a block diagram of another embodiment of aninterrogator constructed in accordance with the principles of thepresent invention,

FIG. 9 illustrates a block diagram of an embodiment of a control andprocessing subsystem constructed in accordance with the principles ofthe present invention,

FIG. 10 illustrates a block diagram of an embodiment of an interrogationsystem demonstrating the capabilities associated with radio frequencyidentification according to the principles of the present invention,

FIG. 11 illustrates a block diagram of an embodiment of a reply codefrom an RFID tag in response to a query by an interrogator constructedaccording to the principles of the present invention,

FIG. 12 illustrates a waveform diagram of an exemplary one-bit cell of aresponse from an RFID tag to an interrogator in accordance with theprinciples of the present invention,

FIG. 13 illustrates a waveform diagram of an exemplary response from anRFID tag in accordance with the principles of the present invention,

FIG. 14 illustrates a waveform diagram of a spectral response associatedwith the response from the RFID tag illustrated in FIG. 13,

FIG. 15 illustrates a waveform diagram demonstrating a representativesignal to noise gain employing an interrogator in accordance with theprinciples of the present invention,

FIG. 16 illustrates a block diagram of an embodiment of aninitialization stage of an interrogation system in accordance with theprinciples of the present invention,

FIG. 17 illustrates a block diagram of an embodiment of a postinitialization stage of an interrogation system in accordance with theprinciples of the present invention,

FIG. 18 illustrates a flow chart of an embodiment of a method ofdetecting an RFID tag according to the principles of the presentinvention,

FIG. 19 illustrates a block diagram of portions of a control andprocessing subsystem of an interrogator constructed according to theprinciples of the present invention,

FIG. 20 illustrates a block diagram of an embodiment of portions of acorrelation subsystem associated with a control and processing subsystemof an interrogator demonstrating an exemplary operation thereof inaccordance with the principles of the present invention,

FIG. 21 illustrates a waveform diagram demonstrating exemplaryadvantages associated with the correlation subsystem described withrespect to FIGS. 19 and 20,

FIG. 22 illustrates a block diagram of an embodiment of a decisionsubsystem associated with a control and processing subsystem of aninterrogator constructed according to the principles of the presentinvention,

FIG. 23 illustrates a block diagram of an embodiment of a correlationsubsystem associated with a control and processing subsystem of aninterrogator constructed according to the principles of the presentinvention,

FIG. 24 illustrates a block diagram of an embodiment of portions of acorrelation subsystem associated with of a control and processingsubsystem of an interrogator constructed according to the principles ofthe present invention,

FIG. 25 illustrates a block diagram of another embodiment of acorrelation subsystem associated with a control and processing subsystemof an interrogator constructed according to the principles of thepresent invention,

FIG. 26 illustrates a block diagram of yet another embodiment of acorrelation subsystem associated with a control and processing subsystemof an interrogator constructed according to the principles of thepresent invention,

FIGS. 27 and 28 illustrate block diagrams of embodiments of Fast FourierTransform operations employable with a correlation subsystem associatedwith the control and processing subsystem of the interrogatorconstructed according to the principles of the present invention,

FIG. 29 illustrates a waveform diagram demonstrating the sidelobesassociated with the correlation subsystem in accordance with theprinciples of the present invention,

FIG. 30 illustrates a block diagram of an embodiment of a predetectingfunction operable with a correlation subsystem associated with a controland processing subsystem of an interrogator constructed according to theprinciples of the present invention, and

FIGS. 31 to 34 illustrate waveform diagrams demonstrating exemplaryperformances of an interrogator according to the principles of thepresent invention.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of the presently preferred embodiments arediscussed in detail below. It should be appreciated, however, that thepresent invention provides many applicable inventive concepts that canbe embodied in a wide variety of specific contexts. The specificembodiments discussed are merely illustrative of specific ways to makeand use the invention, and do not limit the scope of the invention. Thepresent invention will be described with respect to exemplaryembodiments in a specific context, namely, an interrogator, methods ofdiscerning metal objects (i.e., objects that include metal) and RFIDobjects (i.e., objects that include an RFID tag or radio frequencyidentification) and an interrogation system employing the same.

In one aspect, an interrogator constructed according to the principlesof the present invention reads a reply code from an RFID tag and treatsthe reply code as a spreading code in the context of spread spectrumcommunications with the existence of the RFID tag being preferably asingle data bit also in the context of spread spectrum communications.The additional gain achieved by this approach substantially expands apotential for a detection process both of an obstructed RFID tag and anRFID tag located at a greater distance from the interrogator.

In another aspect, an interrogator constructed according to theprinciples of the present invention can read a reply code of the RFIDtag with multiple amplitude bits (e.g., at least two) so that uniquecharacteristics of an amplitude of the reply codes are captured and canbe used to increase the uniqueness associated with the RFID tag. Thus,in addition to a uniqueness of the reply code including a tagidentification (ID) code section thereof, the amplitude informationserves as a type of “fingerprint” for the RFID tag.

In another aspect, the interrogation system is operable in apre-initialization stage, an initialization stage and a postinitialization stage. During one of the pre-initialization stage and theinitialization stage, reference codes corresponding to the reply codesof the RFID tags are logged into memory (e.g., a database) of aninterrogator of the interrogation system. Thereafter and during the postinitialization stage, a correlation subsystem of the interrogatorcorrelates between the reference code and a reply code from the RFIDtags (when subsequently energized by the interrogator) to enhance adetection sensitivity thereof.

In another aspect, an interrogator constructed according to theprinciples of the present invention may employ reference codescorresponding to reply codes for the RFID tags that are syntheticallyderived to attain a higher level of sensitivity therefor. As mentionedherein, amplitude information about an RFID tag can be employed togenerate a reference code to correlate against incoming reply codes todetect the presence of an RFID tag. The amplitude, phase and delay(timing of a response to an excitation signal) information of aparticular type of RFID tag may be employed by the interrogator toderive the synthetic reference code.

One embodiment of generating a synthetic reference code is to pre-load alogical “0” and a logical “1” for a particular class of RFID tags (seeFIG. 12 for an exemplary embodiment of a logical “0” and a logical “1”)wherein the preloaded information contains the unique amplitude, phaseand delay information as mentioned above for that particular type ofRFID tag. The preloaded information is used as individual buildingblocks to construct a synthetic code. During initialization, when theRFID object is typically read under docile signal conditions (i.e., highsignal to noise ratio), so that it can be read in a conventional manner,the complete RFID tag code or digital signature is obtained. Further, aspart of the initialization process, using the complete digital signatureand the preloaded logical “0's” and logical “1's,” a representation ofthe RFID object's response to an RFID query by the interrogator issynthetically constructed one bit at a time, starting with a bit of thedigital signature (a “0” or a “1”) and thereby establishing acorresponding bit of the synthetic code (using logical “0s” and logical“1s”).

In a like manner, another bit of the synthetic code is determined and ifcontiguous to the other bit, is appended to the first bit describedabove. The process continues on a bit by bit basis, until a syntheticcode is generated corresponding to at least a portion of the digitalsignature. In some instances, it will be desired to construct the entireanticipated response of the RFID object synthetically and this inventioncomprehends those instances where the entire RFID response as well asonly a portion of the RFID response is generated synthetically. Thepreload of the logical “1's” and “0's” can occur as part of eachinitialization stage, or they can be contained in non-volatile formwithin the memory of the control and processing subsystem. They willtypically be entered and updated via the communications subsystem of theinterrogator. The interrogator may contain one or more such sets ofthese elements. In addition to creating the reference codessynthetically or partially creating the reference codes synthetically,the interrogator may create the reference code directly by scanning theRFID tag during an initialization stage.

In another aspect, the interrogator constructed according to theprinciples of the present invention employs non-coherent integration ofmultiple sequences of reference codes and reply codes from an RFID tag.In accordance therewith, reference bits (of the reference code) andsample bits (corresponding to the reply code) are latched and correlatedbit-for-bit within a correlator of a correlation subsystem of theinterrogator. A signal from the correlator is processed by a correlationthreshold sense, a threshold of which may be altered to refine athreshold of the correlation operation. An output of the correlationthreshold sense is then input to a summer, which after multipletransmissions and detections is input to a decision subsystem of theinterrogator for an ultimate determination of a presence of an RFID tag.

In another aspect, an interrogator constructed according to theprinciples of the present invention includes a correlation subsystemthat performs a correlation between a reference code and on portions ofa reply code from the RFID tag and provide an indication of a presenceof the RFID tag. For example, correlating on the preamble of the replycode provides a nine decibel increase in detection sensitivity (e.g.,for an eight bit preamble). While employing the preamble alone does notuniquely identify an RFID tag, it does, with a relatively highprobability, indicate the presence of an RFID tag. In accordancetherewith, the reply codes for multiple RFID tags may have the samepreamble and the interrogator correlates a reference code against thepreamble of the reply code to indicate the presence of the RFID tag. Theapriori knowledge of even the preamble further enhances a correlationoperation to increase a sensitivity of the interrogator. Of course, thecorrelation subsystem may also employ a reference code to correlateagainst the entire reply code of the RFID tag depending on theparticular application. Thus, the interrogator employs a discriminatingcorrelation technique to correlate portions of a reply code from an RFIDtag. The discriminating correlation technique may also employ hierarchaldetection techniques to exploit portions of the reply code of an RFIDtag to iteratively detect a presence thereof.

In another aspect, an interrogator constructed according to theprinciples of the present invention includes a decision subsystemembodied in a threshold detector. A threshold detector provides at leasttwo logical outputs, namely, a logical “1” and a logical “0.” Typically,the output of the threshold detector is a predetermined logical “1” or“0” if a threshold criteria is passed, and an alternate logical “0” or“1” if the threshold criteria is not passed. In accordance with theselected criteria, the interrogator can provide, via a user interface,an RFID tag detected, RFID tag not detected, and an indeterminateindication (when at least a third state is available). In the lattercase, a deeper and more thorough detection mode can be employed todetermine the presence of an RFID tag, lack thereof, or an indeterminatecondition. As a result, the interrogator can detect readily availableRFID tags without employing the additional overhead necessary to detectobscured RFID tags.

In another aspect, an interrogator constructed according to theprinciples of the present invention includes a correlation subsystemwith a single correlator or multiple correlators. In the latter case,the multiple correlators can be used to detect multiple RFID tagssimultaneously and, in that instance, the multiple correlators typicallyoperate independently. Alternatively, multiple correlators can beassigned to find a single RFID tag wherein each correlator can be givena slightly different area of time or phase space to search. In otherwords, the multiple correlators are assigned to an RFID tag offset incode space and can search in multiple locations in code space (i.e, bitor chip space as opposed to time space). For this embodiment, a clockingrate greater than the data rate is preferable. The higher rate is oftenreferred to as a chipping rate so that moving plus or minus a chip canbe used to better align the correlators on the data for enhanced signalprocessing gain. The chipping rate generally refers to a higher ratewherein a chip represents a fraction of a bit and, with “J” contiguousbits occupying the same time as one data bit, the chipping rate is “J”times the data rate.

In another aspect, an interrogator constructed according to theprinciples of the present invention includes multiple antennas to obtainadditional gain from diversity. In accordance therewith, differentcorrelators of a correlation subsystem may be assigned to specificantennas. As a result, the interrogator can benefit from spatialdiversity, polarization diversity, angular diversity and time diversity.

In the discussion that follows, the principles of the present inventionwill be described with respect to a medical environment. Those skilledin the art, however, should recognize that the principles of the presentinvention are applicable to other fields such as supply chain managementsystems in the retail industry.

Referring initially to FIG. 1, illustrated is a diagram of an embodimentof a communication system employable in a medical environmentconstructed in accordance with the principles of the present invention.The communication system is configured to distribute, collect andprocess information across a communications network 105 that may includea Local Area Network (LAN), a Wide Area Network (WAN), an Intranet, anExtranet, the Internet, the World Wide Web, the Public SwitchedTelephone Network (PSTN), future extensions of these (e.g., the Internet2), or a combination thereof. For purposes of the present invention, theWorld Wide Web is defined as all the resources and users on the Internetthat are generally using the Hypertext Transfer Protocol (HTTP). In oneembodiment of the present invention, the communication systemcommunicates to each device connected thereto using Transmission ControlProtocol/Internet Protocol (TCP/IP).

TCP/IP is a two-layered protocol. The higher layer, Transmission ControlProtocol (TCP), manages the assembling of a message or file into smallerpackets that are transmitted over the communications network 105 andreceived by a TCP layer that reassembles the packets into the originalmessage. The lower layer, Internet Protocol (IP), handles the addresspart of each packet so that it gets to the right destination. Eachgateway computer on the communication system checks the address todetermine where to forward the message. Even though some packets fromthe same message are routed differently than others, the packets will bereassembled at the destination.

TCP/IP uses the client/server model of communication in which a computeruser (a client) requests and is provided a service (such as sending aWeb page) by another computer (a server) in the communication system.TCP/IP communication is primarily point-to-point, meaning eachcommunication is from one point (or host computer) in the communicationsystem to another point or host computer. TCP/IP and the higher-levelapplications that employ TCP/IP are collectively said to be “stateless”because each client's request is considered a new request unrelated toany previous one (unlike ordinary phone conversations that require adedicated connection for the call duration). Being stateless frees thenetwork paths so that everyone can use the paths continuously. It shouldbe understood that the TCP layer itself is not considered stateless asfar as any one message is concerned; the connection remains in placeuntil all packets in a message have been received.

Internet users are familiar with the even higher layer applicationprotocols that use TCP/IP to get to the Internet. The higher levelapplication protocols include the World Wide Web's Hypertext TransferProtocol (HTTP), the File Transfer Protocol (FTP), Telnet (a command andprotocol that allows users to logon to remote computers), and the SimpleMail Transfer Protocol (SMTP). These and other protocols are oftenpackaged together with TCP/IP.

Personal computer users usually access the Internet through the SerialLine Internet Protocol (SLIP) or the Point-to-Point Protocol (PPP).These protocols encapsulate the IP packets such that the packets can besent over a dial-up phone connection to an access provider's connectiondevice such as a conventional modem.

Protocols related to TCP/IP include the User Datagram Protocol (UDP),the Internet Control Message Protocol (ICMP), the Interior GatewayProtocol (IGP), the Exterior Gateway Protocol (EGP) and the BorderGateway Protocol (BGP). Depending on the circumstance, the UDP may beused instead of TCP for special network communication purposes. Theaforementioned protocols, namely, ICMP, IGP, EGP and BGP, are often usedby network host computers for exchanging router information.

Besides the Internet, TCP/IP may also be employed as the communicationprotocol in the private networks called Intranets and Extranets. AnIntranet is a private network that is contained within an enterprise(such as an organization's office building). The Intranet may consist ofmany interlinked LANs and use leased lines in a WAN. Typically, anIntranet includes connections through one or more gateway computers (notshown) to the outside Internet. The main purpose of an Intranet is toshare organizational information and computing resources amongemployees. An Intranet can also be used to facilitate working in groupsand for teleconferences.

An Intranet typically uses TCP/IP, HTTP and other Internet protocols andin general looks like a private version of the Internet. With tunneling,organizations can send private messages through the public network,using the public network with special encryption/decryption and othersecurity safeguards to connect one part of the Intranet to another.

An Extranet is a private network that uses the Internet protocols andmay use the public network to securely share part of an organization'sinformation or operations with suppliers, vendors, partners, customers,or other medical organizations. An Extranet can be viewed as part of anorganization's Intranet that is extended to users outside theorganization. Just like the Internet, an Extranet also uses HTML, HTTP,SMTP and other Internet protocols.

An Extranet also requires security and privacy provided by the use offirewalls. Firewalls are typically servers that have the ability toscreen messages in both directions so that security is maintained.Firewall servers use digital certificates or similar means of userauthentication, encryption of messages, and the use of virtual privatenetworks (VPNs) that tunnel through the public network.

A medical organization can use an Extranet to exchange large volumes ofdata using Electronic Data Interchange (EDI) and share informationbetween facilities associated therewith. The Extranet can also beemployed to allow an organization to collaborate with otherorganizations on joint development efforts and jointly develop andjointly use training programs. Via the Extranet, an organization canalso provide or access services provided by one organization to a groupof other organizations, such as a medical record management applicationmanaged by one organization on behalf of the medical organization, andshare information of common interest exclusively with partnerorganizations.

Within the medical environment of the communication system is a server110 located at a primary medical facility 120 that includes systems thatallow the server 110 to receive requests, perform specific tasks,retrieve and update information in at least one database and respond torequests sent over the communication system to the server 110. In otherembodiments, the communication system may include multiple servers, eachperforming specific tasks, performing the same tasks, acting asredundant systems or acting as database sites.

In another embodiment of the present invention, the server 110 may be anapplication server. An application server is a computer in a distributednetwork containing specialized programs that provide the business logicfor at least one application program located somewhere within thecommunication system. The application server is frequently viewed aspart of a three-tier application, consisting of a graphical userinterface (GUI) server, an application (business logic) server, and adatabase and a transaction server. The first-tier of the application,also called “front-end,” is usually located in a client computer such asa personal computer (PC) or a workstation and may include a Webbrowser-based graphical user interface. The second-tier is the businesslogic application or set of applications and can be located on a LAN oran Intranet server.

The third-tier of the application, also called “back-end,” is thedatabase and transaction server and may be located on a mainframe or alarge server. Older, legacy databases and transaction managementapplications are part of the back-end or third-tier. The applicationserver is the middleman between the browser-based front-ends and theback-end databases and legacy systems.

In many instances, the application server is combined with or works witha Web server and is called a “Web application server.” The Web browsersupports an easy-to-create HTML-based front-end for the user. The Webserver provides several different ways to forward a request to anapplication server and to send a modified or new Web page to the user.These approaches include the Common Gateway Interface (CGI), FastCGI,Microsoft's Active Server Page (ASP) and the Java Server Page (JSP). Insome cases, the Web application servers also support request “brokering”interfaces such as CORBA's Internet Inter-ORB Protocol (HOP).

The communication system also includes conventional personal computers(PCs) 125, workstations 130, office computer systems 140 and laptopcomputers 150. In other embodiments, the communication system mayinclude any number of PCs 125, workstations 130, office computer systems140 and laptop computers 150. In one embodiment of the presentinvention, the PCs 125, the workstation 130, the office computer system140 and the laptop computers 150 are client computer systems. A clientcomputer system includes a user interface that allows the user to accessinformation, issue requests and perform functions related to the server110. In another embodiment, the office computer system 140 may beconfigured as a second-tier type computer system. For illustrativepurposes only, the PCs 125, the workstation 130, the office computersystem 140 and the laptop computers 150 are located in ones of theprimary medical facility 120, auxiliary medical facility 155 anddoctor's office 160 as shown.

In the illustrated embodiment, the communication system also includes ahandheld device 170 such as a personal digital assistant (PDA) or atablet PC. A PDA is a term used for any small mobile hand-held devicethat provides, in part, computing, information storage and retrievalcapabilities. PDAs are often used for keeping schedules, calendars,address book information and medical information (examples of PDAsinclude Hewlett-Packard's Palmtop™ and 3Com's PalmPilot™). A tablet PCis a compact device similar to a laptop computer but with a handwritingrecognition capability (examples of tablet PCs include Compact TC1000and ViewSonic V1100).

Most PDAs have a small keyboard and some PDAs have an electronicallysensitive pad on which handwriting can be received and recognized.Apple's Newton', which has been withdrawn from the market, was the firstwidely-sold PDA that accepted handwriting. Many applications have beenwritten for PDAs including network programs and Internet accessprograms. PDAs are increasingly combined with telephones and pagingsystems for wireless communications. Some PDAs offer a variation of theMicrosoft Windows™ operating system called Windows CE™. Other PDAproducts use a proprietary operating system, such as PalmOS™ or thirdparty operating systems.

An individually addressable vehicle (IAV) 180 such as an ambulance isalso located within the communication system. The IAV 180 can includeany type of vehicle capable of having a computer with a wireless networkreceiver and/or transmitter that is individually addressable. Forexample, an ambulance containing an Internet terminal is an IAV or acomputer with a wireless receiver/transmitter and sensors that transmitspatient information falls within the class of IAVs.

In the illustrated embodiment, the IAV 180 can send requests to theserver 110 within the communication system to request information orperform specific functions, such as retrieving information related tothe location of the vehicle or general patient information. The IAV 180may include a display (not shown) and an input device (not shown) suchas push buttons, a touch screen or a combination of the two tofacilitate user interface therewith.

A mobile telephone 190 may also be included in the communication system.The mobile telephone 190 includes a display capable of showinginformation retrievable from the communication system. The mobiletelephone 190 can send and retrieve information from the server 110 andperform specialized tasks associated with the capabilities of a mobiletelephone with network capabilities. In one embodiment, the mobiletelephone 190 is capable of accessing Web pages, traversing the Internetand displaying information associated with Web pages on its display.

One skilled in the pertinent art should know that the principles of thepresent invention are not limited for use with the types of devicesdescribed above. In other embodiments, the communication system mayinclude individually accessible electronic devices (IAEDs). IAEDs areelectronic devices having a network interface that are individuallyaddressable on a network. For example, medical equipment in a medicalfacility connected to a network having a unique network address isrepresentative of an IAED.

One skilled in the pertinent art should also know that the principles ofthe present invention may be employed via conventional hardwired orwireless communications networks. For instance, a PDA 170 within theprimary medical facility 120 may communicate patient information to theworkstation 130 via a wireless link compatible with a Bluetoothcommunications environment as defined in the Bluetooth Specification,Version 1.1, or an IEEE 802.11 communications environment as defined inthe Institute of Electronic and Electrical Engineers Specification,Section 802.11, both of which are herein incorporated by reference intheir entirety. Of course, other existing or future wirelessspecifications including those employing a proprietary system may alsobe used. The workstation 130 may then download the information to theserver 110 via a hardwired connection. Of course, the networks withinthe medical facilities and the communications network 105 itself mayinclude hardwired and wireless segments. It should also be clear thatthe principles of the present invention are not limited to acommunication system in a medical environment. For example, theprinciples of the present invention may be employed in conjunction withsupply chain applications for the retail industry.

For a better understanding of communication systems and networks ingeneral, see “Wireless Communications, Principles and Practice,” byTheodore S. Rappaport, Prentice Hall PTR, 1996, “Microwave MobileCommunications,” edited by William C. Jakes, IEEE CommunicationsSociety, 1993, and “Digital Communications,” 3^(rd) Edition, by John C.Proakis, McGraw-Hill, Inc., 1995, all of which are herein incorporatedby reference in their entirety. For a better understanding ofcommunication systems including antenna design and communications, etc.,see “Antenna Engineering Handbook,” by Richard Johnson and Henry Jasik,McGraw-Hill, Inc., 1992, and “Wideband Wireless Digital Communications,”by Andreas F. Molisch, Pearson Education, 2000, which are also hereinincorporated by reference in their entirety.

Turning now to FIG. 2, illustrated is a pictorial diagram of anembodiment of an interrogation system employable within an operatingroom of a medical facility and constructed in accordance with theprinciples of the present invention. In the illustrated embodiment, theinterrogation system is employed within the operating room having anoperating table 202 with a patient 203 thereon and a back table 204 thataccommodates a plurality of surgical instruments 205 a and disposableitems 205 b (such as sponges). Other equipment, such as Mayo stands,ring stands, additional back tables and a kick-bucket are also wellknown to those familiar with the operating room environment and willalso likely be present in some quantity and arrangement. Theinterrogation system includes a computer system 210, an operating roomtransceiver 215 that transmits and receives signals associated with thecomputer system 210 and an interrogator (e.g., a portable interrogator)225. It should be understood that the interrogator may be affixed to atable (e.g., the back table 204), stand, wall or ceiling within afacility and may also be embodied in multiple coordinated systems andsubsystems, both hardware and software.

The computer system 210 may operate as a client that is coupled to aserver associated with the medical facility or, alternatively, thecomputer system 210 may be a stand-alone unit solely dedicated to theoperating room 201. The transceiver 215 is coupled between the computersystem 210 and the portable interrogator 225 and is employed to transmitsignals to and receive signals from the portable interrogator 225.

In the illustrated embodiment, the transceiver 215 includes transmit andreceive sections that are coupled through a wired connection 217 to anantenna array having first, second, third and fourth antenna elements216 a, 216 b, 216 c, 216 d located proximate the corners of theoperating room 201. The antenna array may be employed by the transceiver215 to wirelessly communicate with the portable interrogator 225 throughan interrogator antenna 226 included in the portable interrogator 225.The interrogator antenna 226 may be external as shown, or alternatively,it may be integrated into the body of the portable interrogator 225. Ofcourse, other antenna configurations including additional or fewerantenna elements or alternate placements may be employed as directed byspecific parameters or characteristics associated with an operating roomenvironment. Also, a charging stand 230 which charges the portableinterrogator 225, provides a secure place for the portable interrogator225 to stay when not in use, charges the batteries of the portableinterrogator 225, and can provide an interface between the portableinterrogator 225 and the computer system 210.

The portable interrogator 225 includes a metal sensing subsystem thatprovides a first signal having a metal signature representing a presenceof a metal object and a radio frequency identification (RFID) sensingsubsystem that provides a second signal having an RFID signaturerepresenting a presence of an RFID object. In the illustratedembodiment, the metal sensing subsystem is configured to employ a metalsensing interface and a metal sensing antenna. Similarly, the RFIDsensing subsystem is configured to employ an RFID sensing antennainterface and an RFID sensing antenna. In an alternative embodiment, themetal and RFID sensing subsystems are configured to employ an antennadiplexer and an integrated sensing antenna. Alternatively, the metal andRFID sensing subsystems may be integrated into a sensing subsystem thatprovides a signal or a plurality of signals having at least one of ametal signature representing a presence of a metal object and an RFIDsignature representing a presence of an RFID object.

The portable interrogator 225 also includes a control and processingsubsystem (also referred to as a control and processing system) thatdiscerns a presence of at least one of the metal and RFID objects fromone of the first and second signals. The control and processingsubsystem may also coordinate a processing of the signal(s) from thesensing subsystem to discern a presence of at least one of said metaland RFID objects. The control and processing subsystem may also employan adaptive integrating filter and coordinate a processing of thesignal(s) in conjunction with one of an observable and data to discern apresence of at least one of the metal and RFID objects. The portableinterrogator 225 also includes a communications subsystem, coupled tothe interrogator antenna 226, that allows communication with thetransceiver 215. The portable interrogator 225 further includes aninternal user interface 227 and an external user interface 228 that iscoupled to the computer system 210, in the illustrated embodiment. Theportable interrogator 225 still further includes a position sensor thatallows a position of the portable interrogator 225 to be determined. Theportable interrogator 225 may also have a wired interface, for example,a Universal Serial Bus (USB) port.

Prior to starting a surgical procedure, the portable interrogator 225may be employed to inventory the plurality of surgical instruments 205 aand disposable items 205 b in the operating room 201. This may beaccomplished by scanning the plurality of surgical instruments 205 a anddisposable items 205 b with the portable interrogator 225, beforesurgery begins. Alternatively, a separate asset management system mayprovide this information to the portable interrogator 225. Verificationof the inventory may employ both approaches. In addition, the presenceof a particular kind of surgical instrument 205 a may be verifiedthrough the scanning action as was requested by a medical professional.The request may have been arranged through the computer system 210 orthrough a medical professional's PDA. During the surgical procedure, theportable interrogator 225 may be employed to monitor any movement orrepositioning of the plurality of surgical instruments 205 a anddisposable items 205 b thereby providing location tracking during use ofany of the items in the surgical procedure.

Additionally, the portable interrogator 225 may be employed to scan thepatient 203 before, during or after closing the surgical procedure.Scanning the patient 203 before the surgical procedure provides locationof metal or RFID objects already present in the patient 203. Scanningthe patient 203 during the surgical procedure provides a real-time,operational assurance that the location of the plurality of surgicalinstruments 205 a and disposable items 205 b are where they are intendedto be. Scanning the patient 203 after concluding the surgical procedureprovides a verification that any metal or RFID objects remaining areonly those intended. In addition, the portable interrogator 225 may makemultiple scans about the patient 203 to further assist in ascertaining alocation of any metal or RFID objects.

The portable interrogator 225 may be employed in either an autonomous oran integrated mode of operation. In the autonomous mode of operation,the control and processing subsystem autonomously accomplishes theoperation of the portable interrogator 225, provides all analysisalgorithms and performs all functions needed to discern the presence ofmetal and RFID objects that have been scanned by the portableinterrogator 225. Alternatively, the integrated mode of operationemploys the computer system 210, either wirelessly via the transceiver215, the antenna array and the interrogator antenna 226 or through thewired interface, to support the control and processing subsystem indiscerning the presence of metal and RFID objects. The integrated modemay provide for a greater selection of sensors and sensed items that maybe integrated into an enhanced solution. The integrated mode typicallyallows a more extensive utilization of databases and algorithms to beemployed than in the autonomous mode of operation.

The majority of moveable metal objects employed in the operating room201 are typically surgical instruments or sharps of various sizes andshapes or metal structures intended to be surgically implanted in thepatient 203. Other metal objects may include disposable spongesemploying a metal wire that allows their detection by the metal sensingsubsystem. Each of the metal objects generates a metal signature thatallows its detection by the portable interrogator 225. Generally, themetal signature may cause its associated first signal to possess auniversal characteristic, such as a shape, an amplitude or a frequencyspectrum, that indicates metal is present. More specifically, the metalsignature may cause the first signal to possess a particularcharacteristic that is substantially unique to a particular type ofmetal object thereby allowing a more unique identification.

RFID signatures differ from metal signatures in that the RFID signature,associated with the second signal, is unique and independent of a shapeor a size of the RFID object. The RFID signature is normally provided byan RFID tag (e.g., an RFID information tag including data thereon, ofwhich one is designated 207) applied to the object. For the purposes ofdiscussion herein, a metal object refers to an object including metaland an RFID object refers to an object with an RFID tag or some otherradio frequency identification associated therewith. If an RFID tag wereapplied to a surgical metal instrument, for example, the portableinterrogator 225 would recognize a unique RFID signature as well as themetal signature that may be general or specific to the surgical metalinstrument. The metal signature may be used to discern that an object isa metal object, or that it is a type of surgical metal instrument.However, the RFID signature may be used to discern exactly whichsurgical metal instrument the metal object is. Additionally, RFIDtagging of disposable items, such as sponges, may provide a unique RFIDsignature for each item, whereas disposable items incorporating only ametal wire may typically provide a metal signature specific to all suchitems.

The internal user interface 227 typically includes an integral displayemploying alphanumeric or graphical characters and a touchpad forentering data or information. The internal user interface 227 may alsoemploy audible or visual alarms. In the illustrated embodiment, theexternal user interface 228 includes a monitor 228 a and a keyboard 228b that are wirelessly coupled to the portable interrogator 225 and thecomputer system 210. Alternatively, the wired interface of the portableinterrogator 225 may be employed to couple the external user interface228 to the portable interrogator 225. The external user interface 228may provide a more extensive data entry capability while facilitating abroader monitoring capability than may be provided by the internal userinterface 227.

Position monitoring of the portable interrogator 225 is provided by theposition sensor, which allows a relative determination of its positionwith respect to the patient 203, the back table 204 or another location.RFID position markers may be placed on the patient 203 at predeterminedbenchmark positions, such as the nose, navel, knee and ankle, to providesubstantially unique patient dimensions (or a location of a feature of apatient) and allow other patient attribute positions and metrics to bemore accurately determined. Additionally, RFID/metal calibration markersmay be positioned at other locations on the patient 203 to allow theportable interrogator 225 to calibrate depths or other appropriatethicknesses associated with the patient 203. Of course, RFID markers maybe placed on items such as the back table, ring stand, the Mayo stand orany other location within the operating room deemed appropriate.

A plurality of the portable interrogators 225 may be coupled togethermechanically or electrically, either wirelessly or through their wiredinterfaces, to form a networked-interrogator mode of operation. Thenetworked-interrogator mode of operation allows two or moreinterrogators to share and collaborate data. This collaboration mayinclude the coordination of a plurality of interrogators simultaneouslyemployed on the patient 203. Alternatively, the collaboration may alsoinclude coordinating information associated with the operating room 201,such as information associated with items on the back table 204, as wellas other pertinent information located within the medical facilityenvironment. This collaborative effort may occur in real time or over aperiod of time and may employ the interrogators operating in anautonomous mode, an integrated mode or a combination of the modes.

Turning now to FIG. 3, illustrated is a pictorial diagram of anembodiment of an interrogation system employable within an operatingroom of a medical facility and constructed in accordance with theprinciples of the present invention. In the illustrated embodiment, theinterrogation system is employed within an operating room having anoperating table 302 with a patient 303. Those skilled in the art shouldunderstand, however, that the principles of the present invention areequally applicable to other industries and fields of operation.

The interrogation system includes a computer system 310 coupled to adatabase 311, a transceiver 315 that transmits and receives signalsassociated with the computer system 310 employing a transceiver antenna316 and an interrogator (e.g., a portable interrogator) 325 employing aninterrogator antenna 327. The interrogation system also includes first,second, third and fourth RFID position markers 330, 331, 332, 333,(collectively referred to as RFID position markers 330-333), an RFIDpatient bracelet 340 attached to the patient 303 and first, second,third and fourth RFID/metal calibration markers 341, 342, 343, 344,(collectively referred to as RFID/metal calibration markers 341-344).

The RFID position markers 330-333 may be placed at predeterminedlocations on the patient 303. Generally, RFID position markers provide“position-unique” mapping of the patient 303 wherein the mapping mayemploy more or less RFID position markers than those shown therein. Whenused in conjunction with a sweeping motion of the portable interrogator325, it may be possible to specifically identify the presence of a metalor an RFID tagged object as being located between two of the RFIDposition markers. Thus, a control and processing subsystem of theportable interrogator may employ multiscan, coherent signal processingto coordinate a processing of a plurality of signals (e.g., resultingfrom multiple scans) from a sensing subsystem to discern a presence ofat least one of metal and RFID objects.

Additionally, employing an inertial position sensor in the portableinterrogator 325 allows a more precise determination of the presence andlocation of an RFID or metal object between these two RFID positionmarkers. In this instance, it may be possible to integrate sensor dataacross multiple sweeps of the portable interrogator 325 therebyincreasing its sensitivity and quality of detection. Of course, anynumber of RFID position markers may be employed and positioned asappropriate to a particular situation.

The RFID/metal calibration markers 341-344 have unique RFID signaturesand known amounts and types of metal. They are typically not placed ontop of the patient 303, but are placed beneath or on the side of thepatient 303, such as the underside of a leg or between an arm and thechest. As the portable interrogator 325 is swept over the patient 303,the RFID/metal calibration markers 341-344 are used to calibrate thetype and sensitivity of interrogation needed by the portableinterrogator 325 to provide an acceptable level of object identificationthereby achieving an increased integrity of operation. Of course, anynumber of RFID/metal calibration markers may be employed and positionedas appropriate to a particular situation. Alternatively, calibrationmarkers may be employed that use only RFID or only metal as appropriateto a particular application.

The RFID patient bracelet 340 contains specific information pertainingto the patient 303. The RFID patient bracelet 340 is read, either by theportable interrogator 325 or another appropriate device. The specificinformation may then be applied by the portable interrogator 325 or thecomputer system 310 for the purpose of further improving measurementsensitivity and quality. For example, ample interrogation for a sevenyear old female patient weighing 40 pounds may be quite different fromthat of a 50 year old male patient weighing 260 pounds. Identifying thepatient 303 as well as employing specific databases and generalinformation associated with the patient 303 allows for measurementquality and sensitivity improvements. The computer system 310 employingthe database 311 may be employed in an integrated mode of operation orthe portable interrogator 325 may operate autonomously.

Additionally, the interrogation system may include and cooperate with amedical facility server and extended databases to provide additionalinformation and algorithms to be used as part of the interrogatingprocess. Not only is patient-specific information available, butstatistical information, relevant to patient types may also beavailable. This information may be employed for extensive signalprocessing within the medical facility server, or subsets of thisinformation may be used for signal processing within the computer system310 or the portable interrogator 325 itself.

Turning now to FIG. 4, illustrated is a pictorial diagram of anembodiment of an interrogator (e.g., a portable interrogator)constructed in accordance with the principles of the present invention.The portable interrogator includes an electronics housing 405 having adisplay 406, a touch pad 407 employing a collection of touch keys 408and an audible alarm 409. The portable interrogator also includes asensing antenna assembly 410, a handle 415 and an interrogator antenna420.

The sensing antenna assembly 410 contains RFID and metal sensingantennas and antenna interfaces that may be employed to sense RFID andmetal objects associated with, for instance, a patient in a medicalenvironment. The electronics housing 405 contains a metal sensingsubsystem, an RFID sensing subsystem, a control and processing subsystemand a communications subsystem. The metal sensing subsystem and the RFIDsensing subsystem accept RFID and metal antenna signals, respectively,and are coupled to the control and processing subsystem for signalprocessing that results in the detection of RFID and metal objects, whenpresent. Again, the metal and RFID sensing subsystems may be integratedinto a sensing subsystem. The control and processing subsystem iscoupled to the communications subsystem, which employs the interrogatorantenna 420 to communicate with external computer systems, databases anddisplays. The display 406, the touch pad 407 and the audible alarm 409provide an integral user interface for the portable interrogator. Also,while the interrogators have been described in relation to a medicalenvironment, one skilled in the art should understand that theinterrogator may be employed in other environments and still be withinthe broad scope of the present invention. Also, for further informationabout applications for the interrogation system especially in themedical environment, see U.S. patent application Ser. No. 10/378,043,entitled “An Interrogator and Interrogation System Employing the Same,”filed on Mar. 3, 2003, to Volpi, et al., which is incorporated herein byreference.

Turning now to FIG. 5, illustrated is a system diagram of an embodimentof an interrogator constructed in accordance with the principles of thepresent invention. The interrogator includes a metal sensing subsystem505, a metal sensing antenna interface 506, a metal sensing antenna 507,an RFID sensing subsystem 510, an RFID sensing antenna interface 511, anRFID sensing antenna 512, a control and processing subsystem 515, acommunications subsystem 525, an internal user interface 530, a positionsensor 535 and a power source 540. While the illustrated embodimentprovides for an integrated metal and RFID detection capability, thoseskilled in the art should understand that portions of the interrogatormay be omitted or rendered inactive to provide a metal or RFIDinterrogator.

In the illustrated embodiment, the internal user interface 530 includesa touchpad 531, an integral display 532 and an alarm 533, which mayinclude both an audible alert 533 a and a visual alert 533 b.Additionally, the interrogator employs an external user interface 534(e.g., including a charging stand or cradle), coupled through thecommunications subsystem 525 via a USB connection 526 or a wirelessconnection 527, for example, as shown. The external user interface 534may employ substantially similar elements as the internal user interface530. However, the display and touchpad elements may be larger and moreextensive in capability.

The metal sensing subsystem 505 is coupled to the metal sensing antennainterface 506 and the metal sensing antenna 507 and is configured toprovide a first signal having a signature representing a presence of ametal object. The RFID sensing subsystem 510 is coupled to the RFIDsensing antenna interface 511 and the RFID sensing antenna 512 and isconfigured to provide a second signal having a signature representing apresence of an RFID object. The control and processing subsystem 515 iscoupled to the metal sensing subsystem 505 and the RFID sensingsubsystem 510 and is configured to discern a presence of at least one ofthe metal and RFID objects from one of the first and second signals.

The control and processing subsystem 515 coordinates an operation of themetal sensing subsystem 505 and the RFID sensing subsystem 510.Additionally, the control and processing subsystem 515 analyzes thefirst signal for a metal object signature and the second signal for anRFID object signature. The metal object signature typically may becreated by a change or distortion in a field associated with the metalsensing subsystem 505. The RFID object signature typically may becreated as an identification data sequence associated with an RFIDobject. Analysis of the first and second signals may employ signatureparameters based on factors such as a size, a shape, an orientation,likelihood, a position or a depth associated with the metal object orthe RFID object. Additionally, the analysis may employ data associatedwith metal and RFID objects that is internally or externally stored.

The control and processing subsystem 515 is also coupled to thecommunications subsystem 525, the internal user interface 530 and theposition sensor 535. The position sensor 535 may typically be of aninertial type and may provide either two dimensional (2D) or threedimensional (3D) information as to the position of the interrogator forthe purpose of aiding metal or RFID tag detection. In the illustratedembodiment, the communications subsystem 525 employs transmit andreceive circuitry coupled to an antenna to exchange data with anexternal transceiver.

For example, the communications subsystem 525 may be employed to sendmetal and RFID signature information to an external server for a moreextensive analysis that may be beyond the capability of the control andprocessing subsystem 515. The results of the analysis may be returnedthrough the communications subsystem 525 for final disposition by thecontrol and processing subsystem 515. Alternatively, the control andprocessing subsystem 515 may employ the communications subsystem 525 tosequentially query external databases for signature profiles or analysisalgorithms to be applied locally by the control and processing subsystem515.

The internal user interface 530 allows a user to interact with theinterrogator to provide input and receive output associated with itsapplication. The position sensor 535 cooperates with the control andprocessing subsystem 515 to allow a position of the interrogator to bedetermined. The power source 540 employs a rechargeable or replaceablebattery and provides necessary operating supply voltages to theinterrogator.

The user of the interrogator may employ the touchpad 531 to select amode of operation or both enter and request information about a specificmetal or RFID object. The integral display 532 may be employed to showan RFID number or indicate that the interrogator is detecting thepresence of a metal or RFID object. Alternatively, the external display534 may be employed to indicate the presence of a metal or RFID object.Additionally, the integral display 532 or the external display 534 maybe employed in conjunction with the position sensor 535 to determine aprofile and a position of the metal or RFID object with respect to amovement or sweeping motion of the interrogator. The audible alert 533amay include distinctive tones or synthesized voice communications. Thevisual alert 533b may be flashing or colored features that includetextual or graphical representations. The visual alert 533b may beassociated with the integral display 532, the external display 534, orthey may be stand-alone.

Turning now to FIG. 6, illustrated is a block diagram of anotherembodiment of an interrogator constructed in accordance with theprinciples of the present invention. The interrogator includes a metalsensing subsystem 605, a metal sensing antenna interface 610, a metalsensing antenna 615, an RFID sensing subsystem 620, an RFID sensingantenna interface 630, an RFID sensing antenna 635 and a control andprocessing subsystem 640. While the illustrated embodiment provides foran integrated metal and RFID detection capability, those skilled in theart should understand that portions of the interrogator may be omittedor rendered inactive to provide a metal or RFID interrogator.

The metal sensing subsystem 605 includes a metal sensingdigital-to-analog converter (DAC) 606, a metal sensing transmitamplifier 607, a metal sensing receive amplifier 608 and a metal sensinganalog-to-digital converter (ADC) 609. The metal sensing antennainterface 610 includes a metal sensing transmit conditioning filter 611and a metal sensing receive conditioning filter 612. The metal sensingantenna 615 includes a metal sensing transmit antenna 616 and a metalsensing receive antenna 617.

The RFID sensing subsystem 620 includes an RFID sensing DAC 621, an RFIDsensing transmit selector switch 622, a first RFID sensing transmitamplifier 623, a second RFID sensing transmit amplifier 624, a firstRFID sensing receive amplifier 625, a second RFID sensing receiveamplifier 626, an RFID sensing receive selector switch 627 and an RFIDsensing ADC 628. The RFID sensing antenna interface 630 includes firstand second RFID sensing transmit conditioning filters 631, 632 and firstand second RFID sensing receive conditioning filters 633, 634. The RFIDsensing antenna 635 includes first and second RFID sensing transmitantennas 636, 637 and first and second RFID sensing receive antennas638, 639. “HI band” and “LO band” capabilities are present toaccommodate the wide frequency range necessary to detect the varioustypes of RFID tags.

In an alternative embodiment, a mixing or heterodyning function may beincluded within the RFID sensing ADC 628 or the RFID sensing DAC 621functions. These techniques are known to those skilled in the pertinentart and may be employed to translate signal processing to a moredesirable frequency range thereby allowing less expensive or morereadily available components to be used. Additionally, the specificnature and function of the first and second transmit conditioningfilters 631, 632 and first and second RFID sensing receive conditioningfilters 633, 634 may vary depending on the specific algorithms employedfor control and processing and for signal generation and recovery. Also,some embodiments may not require some or all of the filters shown.

In the illustrated embodiment, the control and processing subsystem 640may be a software defined structure that allows features and functionsof the interrogator to be easily modified or tailored by alteringsoftware functions. The control and processing subsystem 640 employs acrystal oscillator to provide a precise frequency reference for both themetal and RFID sensing subsystems 605, 620.

The control and processing subsystem 640 generates a metal sensingdigital excitation signal based on a metal sensing mode of operationselected and provides this signal to the metal sensing DAC 606. Themetal sensing digital excitation signal may be in the form of acontinuous tone. Alternatively, the digital excitation signal may varyin amplitude, frequency, or phase and may also be of a pulsed naturewherein the waveform duty cycle is less than 100 percent. The frequencyof the metal sensing digital excitation signal may generally be in therange of five to 100 kHz. Different waveforms may be used to optimize adetection of both ferrous and non-ferrous metals. These waveforms may beselected for different sizes and masses of metals and for metals atdifferent locations and depths within a patient. Algorithmic informationemployed in generating these excitation signals may be part of thecontrol and processing subsystem 640.

The metal sensing DAC 606 converts the metal sensing digital excitationsignal into an analog signal that, except for its amplitude, is themetal sensing transmit signal. The analog signal is provided to themetal sensing transmit amplifier 607, which amplifies the analog signalto a correct amplitude for transmission. The output of the metal sensingtransmit amplifier 607 is provided to the metal sensing transmitconditioning filter 611, which sufficiently attenuates all out-of-bandsignals and provides a proper impedance match to the metal sensingtransmit antenna 616. The metal sensing transmit antenna 616 launchesthe metal sensing transmit signal.

A metal object present in the vicinity of the metal sensing transmitantenna 616 and the metal sensing transmit signal will generate a metalsensing return signal wherein the metal sensing return signal may bebased on a change in a field characteristic of the metal sensingtransmit signal. The field characteristic may be altered in the vicinityof the metal object such that a distinctive metal sensing receive signalimpinges on and excites the metal sensing receive antenna 617. Theoutput of the metal sensing receive antenna 617 is provided to the metalsensing receive conditioning filter 612, which sufficiently attenuatesall out-of-band energy and provides a proper impedance match between themetal sensing receive antenna 617 and the metal sensing receiveamplifier 608.

The metal sensing receive amplifier 608 amplifies the metal sensingreceive signal to a level sufficient for processing and provides it tothe metal sensing ADC 609. The metal sensing ADC 609 provides a metalsensing digital signal, proportional to the metal sensing receivesignal, to the control and processing subsystem 640, which determines ifthe metal sensing digital signal has a signature representing a presenceof a metal object in the vicinity of the metal sensing antenna 615.

The control and processing subsystem 640 generates an RFID sensingdigital excitation signal based on an RFID mode of operation selectedand outputs this signal to the RFID sensing DAC 621. The RFID sensingdigital excitation signal may be in the form of a code that excites andenergizes an RFID object present including an RFID tag. The carrierfrequency associated with this code may be in one of two frequencybands. A first frequency band may be centered around 133-135 kHz and isdesignated as the “LO band.” A second frequency band may be centeredaround 10-13 MHz and is designated the “HI band.” Alternatively, a “HIband” around 902-928 MHz may also be employed. Alternatively, the133-135 kHz and the 10-13 MHz bands may be combined in the “LO band” andsome specific implementations may require only a single band. Afrequency band is selected based on the RFID mode of operation selected.Each frequency band corresponds to different types of RFID tags present,which may be based on its size or other factors. Generally, algorithmicinformation to generate the RFID sensing digital excitation signal iscontained in the control and processing subsystem 640.

The RFID sensing DAC 621 converts the RFID sensing digital excitationsignal into an analog signal that, except for amplitude, is the RFIDsensing transmit signal. The RFID sensing transmit signal is provided tothe RFID sensing transmit selector switch 622, which is controlled bythe control and processing subsystem 640. The RFID sensing transmitselector switch 622 directs the RFID sensing transmit signal to thefirst RFID sensing transmit amplifier 623 or the second RFID sensingtransmit amplifier 624, respectively, based on whether the RFID sensingtransmit signal is “HI band” or “LO band.” The first RFID sensingtransmit amplifier 623 and the second RFID sensing transmit amplifier624 increase the amplitude of the “HI band” and “LO band” signals to acorrect amplitude for transmission.

The first RFID sensing transmit amplifier 623 provides the “HI band”signal to the first RFID sensing transmit conditioning filter 631 andthe second RFID sensing transmit amplifier 624 provides the “LO band”signal to the second RFID sensing transmit conditioning filter 632. Thefirst and second RFID sensing transmit conditioning filters 631, 632employ differing center frequencies and sufficiently attenuateassociated out-of-band signals. Additionally, they provide a properimpedance match to their respective first or second RFID sensingtransmit antennas 636, 637, which launch their respective RFID sensingtransmit signals.

An RFID object, including an RFID tag, in the vicinity of the first orsecond RFID sensing transmit antenna 636, 637 generates an RFID sensingreturn signal. The RFID sensing return signal impinges on and excitesthe appropriate first or second RFID sensing receive antenna 638, 639,respectively, to provide an RFID sensing receive signal. An output ofthe first or second RFID sensing receive antenna 638, 639 is provided tothe first or second RFID receive conditioning filter 633, 634,respectively. The first or second RFID receive conditioning filter 633,634 sufficiently attenuates the out-of-band energy and provides a properimpedance match between the first or second RFID sensing receive antenna638, 639 and the first or second RFID sensing receive amplifier 625,626, respectively.

The first or second RFID sensing receive amplifier 625, 626 amplifiesthe small RFID sensing receive signal to a level sufficient forprocessing and provides an amplified RFID sensing receive signal to theRFID sensing receive selector switch 627, which is controlled by thecontrol and processing subsystem 640. The control and processingsubsystem 640 selects the appropriate reception path through the RFIDsensing receive selector switch 627 for input to the RFID sensing ADC628, based on the excitation signal transmitted. The RFID sensing ADC628 provides an RFID sensing digital signal, proportional to the RFIDsensing receive signal, to the control and processing subsystem 640,which determines if the RFID sensing receive signal has a signaturerepresenting a presence of an RFID object in the vicinity of the RFIDsensing antenna 635.

Turning now to FIG. 7, illustrated is a system diagram of an alternativeembodiment of an interrogator constructed in accordance with theprinciples of the present invention. The interrogator includes a metalsensing subsystem 705, an RFID sensing subsystem 710, a metal and RFIDsensing antenna diplexer 712, a metal and RFID sensing integratedantenna 714, a control and processing subsystem 715, a communicationssubsystem 725, a user interface 730, a position sensor 735 and a powersource 740.

The interrogator is similar to the interrogator of FIG. 5 wherein themetal and RFID sensing antenna diplexer 712 has replaced the metalsensing antenna interface 506 and the RFID sensing antenna interface511. Additionally, the metal and RFID sensing integrated antenna 714 hasreplaced the metal sensing antenna 507 and the RFID sensing antenna 512.General operation of the interrogator is also similar to the operationof the interrogator of FIG. 5.

However, the interrogator employs the metal and RFID sensing antennadiplexer 712 between the transmit and receive paths associated with boththe metal sensing and RFID sensing subsystems 705, 710. The metal andRFID sensing antenna diplexer 712 accommodates the frequency selectingand impedance matching functions. Similarly, the metal and RFID sensingintegrated antenna 714 is also employed in both the transmit and receivepaths associated with both the metal sensing and RFID sensing subsystems705, 710. A more detailed discussion of the metal and RFID sensingantenna diplexer 712 and the metal and RFID sensing integrated antenna714 are presented below with respect to FIG. 8.

Turning now to FIG. 8, illustrated is a block diagram of anotherembodiment of an interrogator constructed in accordance with theprinciples of the present invention. The interrogator includes a metalsensing subsystem 805, an RFID sensing subsystem 810, a metal and RFIDsensing antenna diplexer 815, a metal and RFID sensing integratedantenna 825 and a control and processing subsystem 830.

The metal and RFID sensing antenna diplexer 815 includes a metal sensingtransmit conditioning filter 816, a metal sensing receive conditioningfilter 817, a metal sensing diplexer switch 822, first and second RFIDsensing transmit conditioning filters 818, 819, first and second RFIDsensing receive conditioning filters 820, 821 and first and second RFIDsensing diplexer switches 823, 824. The metal and RFID sensingintegrated antenna 825 includes a metal sensing transmit/receive antenna826 and first and second RFID sensing transmit/receive antennas 827,828.

Operation of the metal sensing subsystem 805, the RFID sensing subsystem810 and the control and processing subsystem 830 are analogous to themetal sensing subsystem 605, the RFID sensing subsystem 620 and thecontrol and processing subsystem 640 as was discussed with respect tothe interrogator of FIG. 6. While the RFID sensing subsystem 810includes a single path analog-to-digital converter operating preferablyat least twice the Nyquist rate, it is also well within the broad scopeof the present invention to configure the analog-to-digital conversionprocess to operate at the Nyquist rate by separating the signal to beconverted into in-phase and quadrature phase signals and performingsubsequent processing thereon. Alternatively, the metal and RFID sensingsubsystems 805, 810 may be integrated into a sensing subsystem thatprovides a signal or a plurality of signals having at least one of ametal signature representing a presence of a metal object and an RFIDsignature representing a presence of an RFID object.

However, the interrogator employs the metal and RFID sensing integratedantenna 825, which shares a common antenna between associated transmitand receive signals. For example, the metal sensing transmit/receiveantenna 826 is coupled to both the metal sensing transmit conditioningfilter 816 and the metal sensing receive conditioning filter 817 via themetal sensing diplexer switch 822. Similarly, the first and second RFIDsensing transmit/receive antennas 827, 828 are coupled through the firstand second RFID sensing diplexer switches 823, 824 to theircorresponding RFID sensing conditioning filters.

Generally, the diplexer switches are configured as conventionalthree-port devices so as to provide low loss paths for excitationsignals proceeding from the transmit amplifiers to the antennas andcorrespondingly to provide low loss paths for incoming signals from theantennas to the receiving amplifiers. In time domain configurations,these may be accomplished by simple switching or gating. In continuousmode configurations, this may be accomplished by properly phasingsignals so that they are in-phase when traveling to a desired port andout-of-phase when traveling to an undesired port.

The metal and RFID sensing integrated antenna 825 provides a suitablematch to both launch a transmit signal and accept a receive signalassociated with the metal and RFID sensing functions. The individualantennas may consist of single elements or may themselves be complex innature with multiple elements. The antennas may be partially shielded soas to inhibit transmitted and received radiation to and from unwanteddirections. For example, an antenna being passed over a patient orportion of the operating room such as a back table or MAYO stand, shouldideally have maximum sensitivity in the direction and vicinity of thepatient and maximum attenuation in all other directions.

Turning now to FIG. 9, illustrated is a block diagram of an embodimentof a control and processing subsystem constructed in accordance with theprinciples of the present invention. The control and processingsubsystem includes a digital signal processor 905 employing a frequencycontrol crystal 910, a bootstrap memory 915, a flash memory 920, arandom access memory 925, and an input/output interface 930.

The control and processing subsystem provides the digital signalprocessing functions, the signal generating functions, the controlfunctions and the input and output interface functions associated withan interrogator. The digital signal processor 905 may be embodied as asingle integrated circuit, or as a group of integrated circuitsperforming this role. Additionally, those skilled in the art shouldunderstand that the digital signal processor 905 may be embodied in adigital signal processor engine, combinatorial logic and software, or adigital signal processor engine, an application specific integratedcircuit, glue logic (including level shifters, translators, drivers,synchronizers, etc.) and software, or a special purpose digital signalprocessor engine, general purpose engine and software. The transmitsignals are generated within the digital signal processor 905 which alsoincludes a signal synthesizing function. The frequency control crystal910 provides proper timing for the signal synthesizing function.

The bootstrap memory 915 is a non-volatile read-only memory thatcontains a basic software program to enable the interrogator to powerup, accept commands from the keyboard, display diagnostics, and allowdata ports to be used. In the case of a software-related system failure,the bootstrap system 915 allows the interrogator to recover (i.e.,reboot). It also possesses basic system diagnostics which may be runindependently of whatever software has been loaded.

The flash memory 920 is a non-volatile random access memory, where thecurrent operating system and program of the interrogator is loaded. Thecontents of the flash memory 920 may be changed, updated and checked bydiagnostics and programs contained in the bootstrap memory 915. Forexample, diagnostics exist within the bootstrap memory 915 to test therandom access memory 925. The input/output interface 930 is a portion ofthe control and processing subsystem that accesses all other necessaryportions as well as external ports of the interrogator, wherein thecollection of interface connections 935 may be considered typical.

The data I/O port of the collection of interface connections 935 allowsspecific software releases or upgrades to be loaded into aninterrogator, either wirelessly or using a wireline. In this manner, thecharacteristics of the interrogator may be easily changed or upgraded asnecessary or appropriate. The software defined architecture enables thiscapability since all signal processing decisions and signal generationinitiation occurs within the control and processing subsystem.

Similarly, this architecture permits upgrading of existing algorithmsand incorporation of new algorithms for existing RFID tag and metaldetection without hardware modifications. Therefore, the interrogatormay be employed as substantially a universal interrogator that iscapable of adaptation to read multiple versions of RFID tags fromvarious manufacturers, including future-developed RFID tags. Of course,this may also include metal detection improvements and additions, aswell.

Although the embodiments of this invention presented have concentratedon the detection and monitoring of disposable and non-disposable medicalequipment, alternative embodiments and future applications outside themedical field are envisioned. These applications include integrating thedetection of multiple disparate objects within a single system and theintegration of disparate observables into a single integrating filter.Also included are the real-time integration of observables with multipledatabases, and the real-time offloading of portions of the signalprocessing from the interrogator.

Alternative embodiments may include added detection range, increaseddetection sensitivity in hostile environments, increased detectionintegrity, real-time versatility in dynamically selecting what is to bedetected, and simultaneous universal detection of multiple types of RFIDtags and metallic objects, often operating at disparate frequencies.These applications and embodiments may encompass, for example, inventorymanagement, supply chain management, and security.

Having provided exemplary functionality associated with the interrogatorand interrogation system according to the principles of the presentinvention, the radio frequency identification capabilities willhereinafter be described in greater detail. A protocol independentinterrogation system (e.g., an RFID interrogation system) is describedthat includes at least one RFID excitation source, typically embodied ina transmit function, and a corresponding RFID receive function. Thetransmit and receive functions may be employed in an interrogator thatincludes control and processing subsystems and sensing subsystemsembodied in a software defined architecture wherein a significantportion of signal processing is done in the digital domain after anincoming signal plus any associated noise has been appropriatelydigitized. The interrogator can accurately and predictably detect andread the signals coming from an RFID object (i.e., an object includingan RFID tag or radio frequency identifier) at levels that aresubstantially below the detection threshold of presently availablereaders.

The enhanced sensitivity of the interrogation system according to theprinciples of the present invention can be exploited in severalconfigurations and for several different and disparate applications. Theflexibility of the interrogator allows selected RFID tags to remain“quiet” and others to come alive and “talk.” Additionally, aninterrogator constructed according to the principles of the presentinvention may be configured to operate in a conventional reader mode orto employ enhanced sensitivity to perform a “deep” or “focused”detection mode to search for the RFID tags. By a “deep” or “focused”detection mode, the interrogator may be set to increase an energy of anexcitation signal, command ones of the RFID tags to be “quiet,” increasean integration period of a correlation subsystem of the interrogator,command a user to scan an area more diligently, or increase aid fromexternal sensors (e.g., position sensor). The aforementioned functionsmay be manually set by a user or automatically performed by theinterrogator.

Further with respect to the RFID sensing functionality of theinterrogator, the control and processing subsystem, and RFID sensingsubsystem in conjunction with an antenna transmits radio frequencyexcitation signals and receives a response in the form of a reply codefrom the RFID tag and, when necessary, down converts the responses to alower frequency for digitization purposes. A digital-to-analog converterof the RFID sensing subsystem converts the response to a digital signal.The digitization is typically at least one bit quantization. While theamplitude information is typically not preserved for a one bitquantization, the frequency information of the response is typicallypreserved. As a one bit quantization is subject to squaring loss,quantization employing at least two bits may be preferable depending onthe application. Higher level quantization captures amplitudeinformation that reduces squaring loss and also obtains unique amplitudecharacteristics of specific RFID tags which enhances detectionsensitivity.

The digitized signals are input to a digital signal processor of thecontrol and processing subsystem. The digital signal processing may beenhanced by the use of additional external sensors, for example, aninertial position sensor that can aid detection by noting a movement ofthe interrogator relative to a stationary RFID tag. The control andprocessing subsystem determines a presence or non-presence of an RFIDtag. The systems and subsystems that form the interrogator arecontrolled by the control and processing subsystem including systemmodes, detection modes, and all other relevant functions. A userinterface and communications subsystem are provided for human andcomputer system interaction.

Turning now to FIG. 10, illustrated is a block diagram of an embodimentof an interrogation system demonstrating the capabilities associatedwith radio frequency identification according to the principles of thepresent invention. The interrogation system includes an interrogator1015 including an RFID sensing subsystem 1020 and a control andprocessing subsystem 1030 that energizes an RFID tag 1005 and thenreceives, detects and decodes the encoded RF energy (reflected ortransmitted) from the RFID tag 1005. The control and processingsubsystem 1030 provides overall control of the functions of theinterrogator 1015 as well as any reporting functions. The interrogator1015 may also include a user interface, communications subsystem, apower source and other subsystems as described above.

Additionally, the interrogation system may be employed with multipleRFID objects and with different types of RFID tags. For example, theRFID tags may be passive, passive with active response, and fullyactive. For a passive RFID tag, the transmitted energy provides a sourceto charge an energy storage device within the RFID tag. The storedenergy is used to power a response from the RFID tag wherein a matchingimpedance and thereby a reflectivity of the RFID tag is altered in acoded fashion of ones (“1”) and zeros (“0”). At times, the RFID tag willalso contain a battery to facilitate a response therefrom. The batterycan simply be used to provide power for the impedancematching/mismatching operation described above, or the RFID tag may evenpossess an active transmitting function and may even respond at afrequency different from a frequency of the interrogator. Any type oftag (e.g., RFID tag) whether presently available or developed in thefuture may be employed in conjunction with the interrogation system.Additionally, the RFID objects may include more than one RFID tag, eachcarrying different information (e.g., object specific or sensorsreporting on the status of the object) about the RFID object. The RFIDtags may also include more than one integrated circuit, each circuitincluding different coded information for a benefit of the interrogationsystem.

Turning now to FIG. 11, illustrated is a block diagram of an embodimentof a reply code from an RFID tag in response to a query by aninterrogator constructed according to the principles of the presentinvention. In the present embodiment, the reply code includes threesections, namely, a preamble 1105, a cyclic redundancy check (CRC) 1110to check for bit errors, and a tag identification (ID) code 1115 thatuniquely specifies an RFID tag. In this example, the preamble 1105 is afixed length having eight bits, the CRC 1110 is 16 bits and the tag IDcode 1115 is either 64 or 96 bits. Of course, the length of therespective sections of the reply code and the sections that form thereply code may be modified including the addition of additional ordifferent sections and still fall within the broad scope of the presentinvention.

The interrogator may employ the tag ID code 1115 to more definitivelydetect and identify a specific RFID tag and a digital signatureassociated with the RFID tag. More specifically, it is possible todetect an RFID tag employing portions of or the entirety of the replycode. As an example, the interrogator may employ the tag ID code 1115only to detect a presence of an RFID tag or employ the additional bitsavailable from the CRC 1110 as well as the preamble 1105 or othersections of the reply code to create a longer and more sensitive datastream for processing and identifying an RFID tag. Also, in aconventional reader mode and as noted above, the RFID tags may bedetected via incoming RF energy and without apriori knowledge of anyinformation about the RFID tag. In this instance, a relatively strongsignal incident on the interrogator is preferable to generate asufficiently positive signal to noise ratio (SNR) to reliably detect theincoming signal and, ultimately, the presence of the RFID tag.

Turning now to FIG. 12, illustrated is a waveform diagram of anexemplary one-bit cell of a response from an RFID tag to an interrogatorin accordance with the principles of the present invention. With alogical “1” response, zero encoding is in a frequency shift keying (FSK)modulation format to distinguish logical “1” from logical “0,” but anon/off nature of the backscatter return signal of the RFID tag is alsoactually an amplitude shift keying (ASK) signal. The shift in amplitudeis detected by the interrogator and the frequency of operationdetermines whether the detection represents a logical “1” or logical“0.” For a better understanding of RFID tags, see “Technical Report 860MHz-930 MHz Class I Radio Frequency Identification Tag Radio Frequency &Logical Communication Interface Specification Candidate Recommendation,”Version 1.0.1, November 2002, promulgated by the Auto-ID Center,Massachusetts Institute of Technology, 77 Massachusetts Avenue, Bldg3-449, Cambridge Mass. 02139-4307, which is incorporated herein byreference.

The backscatter return signal is embodied in the response from an RFIDtag. A low backscatter return signal is generated when the RFID tagprovides a matched load so that any energy incident on the antenna ofthe RFID tag is dissipated within the RFID tag and therefore notreturned to the interrogator. Alternatively, a high backscatter returnsignal is generated when the RFID tag provides a mismatched load so thatany energy incident on the antenna of the RFID tag is reflected from theRFID tag and therefore returned to the interrogator. For moreinformation, see “RFID Handbook,” by Klaus Finkenzeller, published byJohn Wiley & Sons, Ltd., 2nd edition (2003), which is incorporatedherein by reference.

Turning now to FIG. 13, illustrated is a waveform diagram of anexemplary response from an RFID tag in accordance with the principles ofthe present invention. The exemplary response includes recordedtransmissions 1315 and backscatter return signals 1320 from the RFID tagunder docile conditions. Under docile conditions, the response from theRFID tag is quite strong and substantially above the ambient noise level1310 and an interrogator can more readily detect the response on anindividual bit-by-bit basis.

Turning now to FIG. 14, illustrated is a waveform diagram of a spectralresponse associated with the response from the RFID tag illustrated inFIG. 13. As illustrated, the spectral response provides a strong signalin accordance with the response from the RFID tag under docileconditions. The signal is essentially in two distinct components. Thefirst component is a strong backscatter return 1410, which is strongestin amplitude and at the center of the response. The second component isthe lower amplitude FSK modulations backscatter return 1415 consistingof a series of peaks. In hostile environments or, more generally, whenthe response from the RFID tag is not as strong such as when the RFIDtag is located at an increased range from the interrogator or the RFIDtag is obstructed from the interrogator by absorptive or reflectivematerials, the backscatter return signals 1415 from the RFID tag to theinterrogator are substantially weakened. Consequently, the detection andidentification of the RFID tag is much more difficult and aninterrogator architecture that can accommodate an improved signal tonoise detection capability under adverse conditions while not increasingthe probability of erroneous responses would be advantageous. As willbecome more apparent, an interrogator and interrogation systemconstructed according to the principles of the present inventionaccommodates reliable identification of the RFID tag under docileconditions and in hostile environments.

By way of example, consider a response from an RFID tag and theexistence thereof to be a one-bit message, namely, the RFID tag iseither present or not. Then, the presence of the RFID tag may be alogical “1” and an absence thereof may be a logical “0,” or vice versa.Then, further consider the bits of the reply code (see, for instance,the reply code of FIG. 11) to be a spreading code for the one-bitmessage. Spreading codes are used in spread spectrum communications toprovide additional gain from signal processing for weak signals. For abetter understanding of spread spectrum technology, see an “Introductionto Spread Spectrum Communications,” by Roger L. Peterson, et al.,Prentice Hall Inc. (1995) and “Modern Communications and SpreadSpectrum,” by George R. Cooper, et al., McGraw-Hill Book Inc. (1986),both of which are incorporated herein by reference.

Further assume that a reference code (representing a reply code orportions thereof such as a tag ID code) is preloaded into aninterrogator and the reply code from the RFID tag plus any noise arecorrelated against the reference code by a correlation subsystem withinthe interrogator. If a match occurs, an increase in a gain [in decibels(dB)] for the matched signal within the interrogator follows therelationships as set forth below: Gain Increase (dB)=10×Log10(N),wherein “N” is the number of bits used in the correlation.

In a numerical example, if an RFID tag with a 64 bit tag ID code is usedfor the correlation, then the gain would be 18.06 dB. Additionally, ifan RFID tag with a 96 bit tag ID code and an eight bit preamble and 16bit CRC is used for the correlation, then the gain would be 20.79 dB.The gain corresponds to an improvement in the SNR as set forth above.

Turning now to FIG. 15, illustrated is a waveform diagram demonstratinga representative signal to noise gain employing an interrogator inaccordance with the principles of the present invention. Typically, areply code associated with a response from an RFID tag is read on abit-by-bit basis and each bit is detected on an individual basis. Underless than ideal conditions, the detected bits 1510 are often onlyslightly above an ambient noise level 1525 as illustrated in FIG. 15.Preferably, an interrogator constructed according to the presentinvention reads the entire reply code and treats the reply code as aspreading code with the existence of the RFID tag being only a singledata bit. Thus, the additional gain 1515 achieved by this approachproduces a substantially larger signal 1505 which substantially expandsa potential for a reliable detection process both of an obstructed RFIDtag and with an RFID tag located at a greater distance from theinterrogator.

The detection threshold 1520 is a level that should be sufficientlyabove noise so that false alarms do not readily occur such that autility of the interrogator is defeated. Correspondingly, setting thedetection threshold 1520 too high will detrimentally affect thesensitivity of the interrogator. Thus, calibrating the threshold of theinterrogator provides more latitude in setting a threshold withoutadversely affecting a detection sensitivity thereof. The additionaldetection sensitivity may be advantageously used in several ways. Forexample, the additional detection sensitivity can be used to increasedetection range to detect RFID tags when obscured by attenuatingmaterials, or to substantially improve the statistical quality of adetection (or a non-detection). Thus, by correlating the reference codewith incoming reply codes from RFID tags plus ambient noise, thepresence of the RFID tags can be reliably detected when the reply codesfrom the RFID tags are at substantially lower signal levels than ispresently possible.

Turning now to FIG. 16, illustrated is a block diagram of an embodimentof an initialization stage of an interrogation system in accordance withthe principles of the present invention. During an optionalpre-initialization stage, reference codes corresponding to the replycodes of the RFID tags 1605 on the RFID objects 1610 (designated A, B,C, . . . N) are logged into memory (e.g., a database) of an interrogator1615 of the interrogation system. Then, during an initialization stage,the RFID objects 1610 are scanned by the interrogator 1615 at which timeeach individual reply code of the RFID tags 1605 is read and checkedagainst the database of reference codes logged during thepre-initialization stage. If a pre-initialization stage is notperformed, the reference codes corresponding to the reply codes of theRFID tags 1605 are logged into the database during the initializationstage.

During the initialization stage, the RFID tags 1605 are not typicallyobstructed so that identifying the reply codes of the RFID tags 1605 canbe performed without employing a high level of sensitivity. When read inthis manner, the tag ID code and other fields (which may serve as thereference code) corresponding to the reply code of the RFID tag 1605 canbe captured using multiple amplitude bits (e.g., at least two) so thatunique characteristics of an amplitude of the reply codes are alsocaptured and can be used to increase the uniqueness associated with theRFID tag 1605 and employable during a subsequent correlation operation.While single bit detection is also possible and within the broad scopeof the present invention, a single bit detection may be subject to asensitivity degradation of a squaring loss thereof. The interrogator1605 may also capture characteristics associated with the environmentsuch as ambient background noise to further increase the sensitivity ofthe interrogator 1605.

There are several alternative methods to create the reference code. Bydirectly scanning the RFID tag 1605 during the initialization stage, themost information about the RFID tag 1605 can be obtained to form thereference code. Alternatively, the reference code can be derivedsynthetically. To derive the reference code synthetically, theamplitude, phase and delay (timing of a response to an excitationsignal) information of a particular type of RFID tag 1605 may beemployed by the interrogator 1615 to derive the synthetic referencecode. Another technique to derive the reference code is also asynthetically derived reference code employing one bit quantization.While the amplitude information may not be preserved for a one bitquantization, the frequency information of the response is typicallypreserved. As a one bit quantization is subject to squaring loss,quantization employing at least two bits may be preferable depending onthe application. Higher level quantization captures amplitudeinformation that reduces squaring loss and also obtains unique amplitudecharacteristics of specific RFID tags 1605 which enhances detectionsensitivity.

In an alternative embodiment, the reference code corresponding to thereply code for the RFID tags 1605 can be input digitally via a keyboardor via a data port and a correlation pattern may be generated in thedigital domain using one bit for each data bit. While amplitudeinformation is not typically captured or used under these circumstances,the ambient noise information can still be collected to augment thesensitivity of the interrogator 1615. In yet another related embodiment,preloaded amplitude information may be input into the interrogator 1615that matches amplitude characteristics of a reply code of an RFID tag1605. As a result, a sensitivity of the interrogator 1615 is enhancedwhile, at the same time, taking advantage of digitally inputting thereference code corresponding to the reply code of the RFID tag 1605.Thereafter and during a post initialization stage, the correlationsubsystem of the interrogator 1615 correlates between the reference codeand a reply code from the RFID tags 1605 (when subsequently energized bythe interrogator 1615) to enhance a detection sensitivity thereof.

Turning now to FIG. 17, illustrated is a block diagram of an embodimentof a post initialization stage of an interrogation system in accordancewith the principles of the present invention. In an exemplary manner, aninterrogator 1720 is shown scanning across first, second and third setsof tagged items 1705, 1710, 1715 (also referred to as RFID objects).Here, the first set of tagged items 1705 represents unobstructed tags,while the second and third sets of tagged items 1710, 1715 representobstructed, obscured, or more distant tags. In general, scanningunobstructed RFID tags can be read by an interrogator 1720 withoutemploying a high level of sensitivity and may be read at substantialranges (e.g., 15-20 feet). Under such circumstances, the interrogator1720 may not employ a correlation operation. In contrast, scanningobstructed RFID tags (by an obstructing media or even at greater ranges)is preferably performed by an interrogator 1720 employing a higher levelof sensitivity. Under such circumstances, the interrogator 1720 mayemploy a correlation subsystem to correlate between a reference code(previously logged) and a reply code for an RFID tag.

In such instances, the interrogator 1720 may correlate on small portionsof a reply code from the RFID tag and provide an indication of apresence of the RFID tag. For example, correlating on the preambleprovides a nine decibel increase in detection sensitivity when thepreamble is eight bits long. While employing the preamble alone does notuniquely identify an RFID tag, it does, with high probability, indicatethe presence of an RFID tag. Of course, the correlation subsystem mayalso employ a reference code to correlate against the entire reply codeof the RFID tag depending on the particular application. Other detectiontechniques such as non-coherent averaging may be employed by theinterrogator to uniquely identify an RFID tag.

Turning now to FIG. 18, illustrated is a flow chart of an embodiment ofa method of detecting an RFID tag according to the principles of thepresent invention. The system is first powered up performing a systemboot and setup 1805. This is typically followed by a preinitializing andinitializing stage 1810. As mentioned above, during an initializationstage reference codes corresponding to the reply codes of the RFID tagare logged into the interrogator. The reference codes corresponding tothe reply codes of the RFID tags are read by or logged into theinterrogator, noting specific RFID tag characteristics along with RFIDtag's ID code. This is typically done by loading the reference codesinto memory and establishing a log within the interrogator during a loadmemory stage 1815. Thereafter and during the post initialization stage,the interrogator scans for the RFID tags during a scan stage 1820 (whichmay be a single scan or non-coherent integration) and, via the replycode, detects and logs the detection thereof during a detection stage1825 (which may be located, individually and in groups, correlated witha log).

A correlation operation performed by the interrogator correlates betweenthe reference codes and the reply codes from the RFID tags. Anyreference codes previously logged, but not matched during thecorrelation operation, are flagged during a flagging stage 1830 via auser interface of the interrogator to the user (including audible queuesor alarms), communications subsystem to a computer system, or othersuitable techniques. Via alerts, the search is repeated during a repeatsearch stage 1835 with added sensitivity until the reference codescorresponding to the reply codes of previously logged RFID tags matchthe reply codes from the RFID tags detected during the postinitialization stage. Returning to the interrogator in search mode, thisis typically a first attempt at locating an RFID tag. When the searchesare repeated, other approaches may be employed including multiple scans,highly localized scanning, or coherent integration as discussed later.

Turning now to FIG. 19, illustrated is a block diagram of portions of acontrol and processing subsystem of an interrogator constructedaccording to the principles of the present invention. The control andprocessing subsystem includes a correlation subsystem 1905 and adecision subsystem 1910. While in the illustrated embodiment thecorrelation subsystem and the decision subsystem form a portion of thecontrol and processing subsystem such as within a digital signalprocessor thereof, those skilled in the art should understand that thesubsystems may be discrete subsystems of the control and processingsubsystem of the interrogator.

As mentioned above, the interrogator may employ a correlation operationto correlate between reference codes (generally designated 1915)corresponding to reply codes (generally designated 1925) from the RFIDtags and subsequently received and digitized reply codes from the RFIDtags to enhance a sensitivity of the interrogator. The reply codes aretypically generated as complex I+jQ signals where I signifies the inphase portion of the signal and Q signifies its quadrature counterpart.The reference codes may be scanned in during the initialization stage orderived synthetically as hereinafter described. To derive the referencecode synthetically, the amplitude, phase and delay (timing of a responseto an excitation signal) information of a particular type of RFID tagmay be employed by the interrogator to derive the synthetic referencecode. The correlation occurs in a correlator 1930 wherein the reply codeis correlated with the reference code. The correlation is mathematicallyanalogous to a convolution operation. For a better understanding ofconvolution theory, see “An Introduction to Statistical CommunicationTheory,” by John B. Thomas, published by John Wiley & Sons, Inc. (1969),which is incorporated herein by reference.

A stream of incoming data to the interrogator (corresponding toresponses in the form of reply codes from the RFID tags) is correlatedagainst preloaded reference codes loaded into a reference code databasein time. Alternatively, samples of the incoming data may be gated in ablock by the interrogator and then the data is correlated in blockmanner against the reference codes. In the latter example, a gatingprocess is employed to gate the incoming data properly. Under suchcircumstances, it is preferable that apriori knowledge of a timing ofthe responses from the RFID tag in connection with a query by theinterrogator better serves the process of gating the block of incomingdata (i.e., the responses) from the RFID tags. Any known delay in theresponses from the RFID tags can be preloaded in the interrogator duringthe initialization stage. An external sensor such as a position sensor(e.g., inertial sensor) may be employed by the interrogator to aid thecorrelation subsystem in predicting the timing of a response from theRFID tag. A synchronization pulse (derived from the transmit excitationsignal) may also be employed to better define a timing of a responsefrom an RFID tag.

The output of the correlator 1930 representing individual correlationsof the reference code with incoming data is summed in a summer 1935providing a correlation signal to improve the signal to noise ratio ofthe correlated signal. The correlation signal from the summer 1935 istypically input into a threshold detector 1940 within the decisionsubsystem 1910 to verify a presence of an RFID tag. The thresholddetector 1940 typically compares the correlation signal with at leastone threshold criteria or value (also referred to as threshold). Thethreshold may be fixed or dynamically determined. In one exemplaryembodiment, where only a single threshold is present, an RFID tag isdeclared present if the correlation signal from the summer 1935 exceedsthe threshold, and not present if the converse is true. In otherembodiments, multiple thresholds may be used to indicate various levelsof probabilities as to the likelihood that an RFID tag is present ornot. This information may then be used to initiate selected oradditional search modes so as to reduce remaining ambiguities.

Regarding the timing of the responses from the RFID tag, a tracking ofthe reply codes may suggest that the reply code is early, prompt orlate. If the tracking suggests that the reply code is prompt (promptoutput greater than early and later output), then a gating function isproperly aligned to provide a significant correlator output. If thetracking suggests that the reply code is early, then the earlycorrelator output is significant as compared to the late correlatoroutput and the correlation subsystem 1905 is tracking too early and therequisite adjustment may be performed. An opposite adjustment may beperformed if the tracking suggests that the reply code is late.

Another approach is to use a tracking loop that uses past successfuldetection performance to establish a gating process for subsequentcorrelations. In yet another embodiment relating to the correlation ofthe reply codes from the RFID tags is to perform Fast Fourier Transforms(FFTs) on both the reference code and a gated sample of the reply codesfrom the RFID tags. Then, a convolution operation in “Fourier Space” maybe performed employing the convolution theorem. The convolution theoremstates that the convolution of two functions is the product of theFourier transforms thereof. An output of the correlation operation istypically envelope detected and several outputs may be averaged in asumming operation that preserves time characteristics of each individualdetection.

Turning now to FIG. 20, illustrated is a block diagram of an embodimentof portions of a correlation subsystem associated with a control andprocessing subsystem of an interrogator demonstrating an exemplaryoperation thereof in accordance with the principles of the presentinvention. In the present embodiment, a technique referred to as a“corner turning memory” is used in accordance with the correlationsubsystem allowing a summing and averaging process for multiplecorrelations. An output of a correlator is read into memory by rows (oneof which is designated 2010) with each row designating a singlecorrelation. Then an output from the summing process (which embodies thememory or a function thereof) is generated by summing across individualcolumns (generally designated 2015, hence the name corner turning)applying an appropriate scaling factor. An output from the memoryrepresents an average of “P” outputs of the correlation subsystemwherein “P” is the number of rows in the corner turning memory. Assuminga signal is located in every row of the corner turning memory, theimprovement in SNR is increased by the square root of “P.”

Using this approach, several options for enhancing performance of theinterrogator are possible. For example, the results of differentaveraging times can be almost simultaneously compared and the modes ofoperation of the interrogator adjusted for enhanced performance. Also,this approach allows the sliding average technique (as described above)to be employed so that the output from the memory is an average over apredetermined period of time. Also, other averaging techniques inaddition to the use of the corner turning memory are also well withinthe broad scope of the present invention.

Turning now to FIG. 21, illustrated is a waveform diagram demonstratingexemplary advantages associated with the correlation subsystem describedwith respect to FIGS. 19 and 20. In the illustrated embodiment, aconventional waveform 2115 represents the probability of detection for agiven carrier to noise ratio (C/No) of a conventional reader reading atleast one out of 100 possible attempts. A total of 100 trials wereaveraged in accordance with the correlation subsystem and an improvedwaveform 2110 represents the increased probability of detection for agiven C/No of an interrogator thereby demonstrating an improvement inSNR of 28.06 dB. This represents 18.06 dB due to correlation operationwherein a length 64 electronic product code (EPC) code was used, plus anadditional 10 dB due to non-coherent averaging. A purpose of thecorrelation operation is to determine whether or not the output oraveraged output of the interrogator represents a presence of an RFIDtag. A threshold detector as herein described then interprets acorrelation signal from the correlation subsystem and provides adecision if the output is of sufficient quality to indicate if an RFIDtag is present or not and, if indeterminate, to perform a “deeper” ormore “focused” search.

Turning now to FIG. 22, illustrated is a block diagram of an embodimentof a decision subsystem associated with a control and processingsubsystem of an interrogator constructed according to the principles ofthe present invention. In the illustrated embodiment, the decisionsubsystem is referred to as a threshold detector as it indicates astatus on a correlation signal input from a correlation subsystem . Athreshold detector 2220 provides at least two logical outputs, namely, alogical “1” and a logical “0.” The output of the threshold detector maybe a predetermined logical “1” or “0” if a threshold criteria is passed,and an alternate logical “1” or “0” if the threshold criteria is notpassed. In short, the output is YES if an RFID tag is detected meaningthat the input exceeds a threshold and NO if an RFID tag is not detectedmeaning that the input does not exceed the threshold.

A decision threshold for the threshold criteria may be embodied inseveral different ways as set forth below: 1. the threshold criteria maybe a predetermined fixed value, 2. the threshold criteria may be a timevarying value based on multiple efforts and averaging algorithms used todetect an RFID tag, 3. the threshold criteria may be related to thenumber of rows used for averaging in the corner turning memory asdescribed above, 4. the threshold criteria may be influenced by ambientconditions (for example, noise) detected in the environment during theinitialization stage as described above, 5. the threshold criteria maybe determined by a tolerated level of false alarms or missed detections,or 6. the threshold criteria may be determined by whether a cursory orvery extensive (“deep” or “focused”) search has been directed by thecontrol and processing subsystem. Other threshold criteria are clearlywithin the broad scope of the present invention. Also, a noise meter maybe employed within the threshold criteria for the threshold detector2220.

Additionally, more than two outputs may also be present to indicatestates of the presence or non-presence of the RFID tag and may existwith lower criteria. For example, an indication may be provided that anRFID tag “may” be present, however the statistical criteria for falsealarm may not have been met. This information may be useful andpresented in a user interface (e.g., a display or data output), or itmay also be used by the control and processing subsystem to change oradjust an operating mode in the interrogator to resolve the ambiguityand direct subsequent actions.

Turning now to FIG. 23, illustrated is a block diagram of an embodimentof a correlation subsystem associated with a control and processingsubsystem of an interrogator constructed according to the principles ofthe present invention. A selected number of reference bits R1 . . . RNassociated with reference codes corresponding to reply codes of RFIDtags are latched and fed into one input of a correlator 2315 one bit ata time. A stream of derived sample bits T1 . . . TN (corresponding tosample bits S1 . . . SQ of the reply codes from the RFID tags) are inputto the correlator 2315 at substantially the same time. A gate pulse froma gating clock synchronizes the inputs to the correlator 2315 at theproper time. Additionally, a Q to N operation 2325 (where “Q” is thelength of the shift register 2335 that receives the sample bits S1 . . .SQ and “N” is the size of a latch 2345) is performed wherein the derivedsample bits T1 . . . TN from the sampled bits S1 . . . SQ are input intoa latch 2345 of the correlation subsystem. Bits reaching the output ofthe shift register 2335 are typically discarded.

If time synchronization between the reference bits R1 . . . RN and thederived sample bits T1 . . . TN is correct, a signal from the correlator2315 will be highly correlated resulting in a large output signal to thecorrelation threshold sense 2350. Also, the output signal will vary inamplitude for the derived sample bits T1 . . . TN corresponding to thebits of the reply code that are before and after a high level ofcorrelation. If time synchronization is not proper or if the reply codeis not present, then the correlation function will provide a noisyoutput. The gating clock is typically shiftable in time and phase tosearch for a proper alignment between the two signals. To achieve properalignment, a correlation subsystem employing a single correlator cansearch across time and phase with respect to the transmission of anexcitation signal. Alternatively, a correlation subsystem that employsmultiple correlators can be set to slightly different time and phasesettings to simultaneously search across a larger phase and frequencyspace.

The correlation threshold sense 2350 is similar to the thresholddetector illustrated and described with respect to FIG. 22. A thresholdof the correlation threshold sense 2350 is typically set quite low toallow for a reasonable averaging to occur in subsequent processing. Inthis context, the correlation threshold sense 2350 may be considered apre-filter to a decision subsystem. In an alternative embodiment, thecorrelation threshold sense 2350 may be performed in the decisionsubsystem. An output of the correlation threshold sense 2350 is theninput to a summer 2355, which after multiple transmissions anddetections (non-coherent integration) provides a correlation signal tothe decision subsystem.

In an exemplary embodiment, the correlation threshold sense 2350receives an output from an exclusive OR function that performs abit-by-bit operation on the reference bits R1 . . . RN and the derivedsample bits T1 . . . TN. Multiple outputs of the exclusive OR functionare accumulated in the correlation threshold sense 2350 and compared toa threshold value. If the outputs of the exclusive OR function exceedthe threshold value, a relatively good indication is provided of alocation of a peak of a correlation triangle. Otherwise, a relativelygood indication is provided that the peak of the correlation triangleeither is outside of the sampled bits S1 . . . SN or that the peak ofthe correlation triangle is not substantially centered within thesampled bits S1 . . . SN. The accumulated outputs of the exclusive ORfunction are thereafter transmitted to the summer 2355 which may includea corner turning memory (see FIG. 20) for further processing. In theenvironment of the corner turning memory, each accumulated output of theexclusive OR function represents a data bit (such as D(1, P)) of a rowof the corner turning memory.

An exemplary correlation triangle 2365 in reference to a threshold valueis illustrated in FIG. 23. Within the correlation threshold sense 2350,a threshold detector provides a high signal when the accumulated outputsof the exclusive OR function exceed a threshold value 2370 and,otherwise, provide a low signal. A resulting pulse or signal 2375 asillustrated in FIG. 23 may be employed to ascertain (e.g., estimate) alocation of a peak of the correlation triangle 2365.

Turning now to FIG. 24, illustrated is a block diagram of an embodimentof portions of a correlation subsystem associated with a control andprocessing subsystem of an interrogator constructed according to theprinciples of the present invention. In particular, the presentembodiment demonstrates a Q to N operation, wherein “Q” is the length ofa shift register 2410 that receives the sample bits S1 . . . SQ of thereply codes from the RFID tags and “N” is the size of a latch. Theincoming data including the reply code from the RFID tags is sampled ata rate higher than the signal rate of the reply code to generate thesampled bits S1 . . . SQ. Then the Q to N conversion occurs by summingsuccessive consecutive sequences of J bits, where Q=J×N and “J” is thenumber of sampled bits in a sequence. As a result, derived sample bitsT1 . . . TN are derived from the sample bits S1 . . . SQ. Appropriatescaling factors are then applied to the derived sample bits T1 . . . TNand the reference bits to account for the summing operation.

Turning now to FIG. 25, illustrated is a block diagram of anotherembodiment of a correlation subsystem associated with a control andprocessing subsystem of an interrogator constructed according to theprinciples of the present invention. In contrast to other embodiments ofthe correlation subsystem wherein a length of the incoming dataincluding the sample bits S1 . . . SQ of the reply codes from the RFIDtags is decreased from Q to N (see description with respect to FIG. 24above), in the present embodiment the number of reference bits R1 . . .RN is increased to provide a set of derived reference bits U1 . . . UQfrom a N to Q operation 2510. The N to Q operation 2510 is substantiallyopposite to the Q to N operation illustrated and described with respectto FIG. 24. An output of the N to Q operation is then input to acorrelator 2525. Additionally, the correlation subsystem employs ahigher sampling rate with output results (after application of theproper scaling factors) analogous to the correlation subsystemillustrated and described with respect to FIG. 23.

A gate pulse from a gating clock synchronizes the inputs to thecorrelator 2525 at the proper time. Additionally, derived sample bits T1. . . TQ are latched into a Q bit latch 2530 of the correlationsubsystem. These bits are a result of sample bits of a reply codeclocking through a shift register 2540. Bits reaching the output of theshift register 2540 are typically discarded.

As in FIG. 23, if time synchronization between the derived referencebits U1 . . . UQ and the derived sample bits T1 . . . TQ is correct, asignal from the correlator will be highly correlated resulting in alarge output signal to the correlation threshold sense 2550. Also, theoutput signal will vary in amplitude for the sample bits T1 . . . TQcorresponding to the bits of the reply code that are before and after ahigh level of correlation. If time synchronization is not proper or ifthe reply code is not present, then the correlation function willprovide a noisy output. The gating clock is typically shiftable in timeand phase to search for a proper alignment between the two signals. Toachieve proper alignment, a correlation subsystem employing a singlecorrelator can search across time and phase with respect to thetransmission of an excitation signal. Alternatively, a correlationsubsystem that employs multiple correlators can be set to slightlydifferent time and phase settings to simultaneously search across alarger phase and frequency space.

The correlation threshold sense 2550 is similar to the thresholddetector illustrated and described with respect to FIG. 22. A thresholdof the correlation threshold sense 2550 is typically set quite low toallow for a reasonable averaging to occur in subsequent processing. Inthis context, the correlation threshold sense 2550 may be considered apre-filter to the decision subsystem. In an alternative embodiment, thecorrelation threshold sense 2550 may be performed in the decisionsubsystem. An output of the correlation threshold sense 2550 is theninput to a summer 2555, which after multiple transmissions anddetections (non-coherent integration) provides a correlation signal to adecision subsystem.

In an exemplary embodiment, the correlation threshold sense 2550receives an output from an exclusive OR function that performs abit-by-bit operation on the derived reference bits U1 . . . UQ and thederived sample bits T1 . . . TQ. Multiple outputs of the exclusive ORfunction are accumulated in the correlation threshold sense 2550 andcompared to a threshold value. If the outputs of the exclusive ORfunction exceed the threshold value, a relatively good indication isprovided of a location of a peak of a correlation triangle. Otherwise, arelatively good indication is provided that the peak of the correlationtriangle either is outside of the sampled bits S1 . . . SQ or that thepeak of the correlation triangle is not substantially centered withinthe sampled bits S1 . . . SQ. The accumulated outputs of the exclusiveOR function are thereafter transmitted to the summer which may include acorner turning memory (see FIG. 20) for further processing. In theenvironment of the corner turning memory, each accumulated output of theexclusive OR function represents a data bit (such as D(1, P)) of a rowof the corner turning memory.

An exemplary correlation triangle 2565 in reference to a threshold value2570 is illustrated in FIG. 25. Within the correlation threshold sense2550, a threshold detector provides a high signal when the accumulatedoutputs of the exclusive OR function exceed the threshold value 2570and, otherwise, provide a low signal. A resulting pulse 2575 asillustrated in FIG. 25 may be employed to ascertain (e.g., estimate) alocation of a peak of the correlation triangle 2565.

Turning now to FIG. 26, illustrated is a block diagram of yet anotherembodiment of a correlation subsystem associated with a control andprocessing subsystem of an interrogator constructed according to theprinciples of the present invention. The reference code corresponding toa reply code of an RFID tag is captured during an initialization stageand stored as a higher number of reference bits R1 . . . RQ. These bitsare then latched into a Q-bit latch 2610 as derived reference bits U1 .. . UQ and fed into one input of a correlator 2620 one bit at a time. Astream of derived sample bits T1 . . . TQ (corresponding to sample bitsS1 . . . SQ of the reply codes from the RFID tags) are input to thecorrelator 2620 at substantially the same time. The correlationoperation is then performed without employing the Q to N or N to Qconversion operation as set forth above. Performing the initializationstage with a higher number of bits (at the higher bit rate) capturesmore of the unique characteristics of the RFID tag and can be used tofurther aid the correlation subsystem. The aforementioned correlationsubsystems perform a non-coherent integration of multiple samples (e.g.,100 samples). A gate pulse from a gating clock synchronizes the inputsto the correlator 2620 at the proper time. Additionally, a Q-bit latch2625 is used to latch the derived sample bits T1 . . . TQ into a latch2625. Bits reaching the output of a shift register 2635 are typicallydiscarded.

If time synchronization between the reference bits R1 . . . RQ and thesample bits T1 . . . TQ is correct, a signal from the correlator 2620will be highly correlated resulting in a large output signal to acorrelation threshold sense 2645. Also, an output signal will vary inamplitude for the derived sample bits T1 . . . TQ corresponding to thebits of the reply code that are before and after a high level ofcorrelation. If time synchronization is not proper or if the reply codeis not present, then the correlation function will provide a noisyoutput. The gating clock is typically shiftable in time and phase tosearch for a proper alignment between the two signals. To achieve properalignment, a correlation subsystem employing a single correlator cansearch across time and phase with respect to the transmission of anexcitation signal. Alternatively, a correlation subsystem that employsmultiple correlators can be set to slightly different time and phasesettings to simultaneously search across a larger phase and frequencyspace.

The correlation threshold sense 2645 is similar to the thresholddetector illustrated and described with respect to FIG. 22. A thresholdof the correlation threshold sense 2645 is typically set quite low toallow for a reasonable averaging to occur in subsequent processing. Inthis context, the correlation threshold sense 2645 may be considered apre-filter to the decision subsystem. In an alternative embodiment, thecorrelation threshold sense 2645 may be performed in the decisionsubsystem. An output of the correlation threshold sense 2645 is theninput to a summer 2650, which after multiple transmissions anddetections (non-coherent integration) provides a correlation signal to adecision subsystem.

In an exemplary embodiment, the correlation threshold sense 2645receives an output from an exclusive OR function that performs abit-by-bit operation on the derived reference bits U1 . . . UQ and thederived sample bits T1 . . . TQ. Multiple outputs of the exclusive ORfunction are accumulated in the correlation threshold sense 2645 andcompared to a threshold value. If the outputs of the exclusive ORfunction exceed the threshold value, a relatively good indication isprovided of a location of a peak of a correlation triangle. Otherwise, arelatively good indication is provided that the peak of the correlationtriangle either is outside of the sampled bits S1 . . . SQ or that thepeak of the correlation triangle is not substantially centered withinthe sampled bits S1 . . . SQ. The accumulated outputs of the exclusiveOR function are thereafter transmitted to the summer 2650 which mayinclude a corner turning memory (see FIG. 20) for further processing. Inthe environment of the corner turning memory, each accumulated output ofthe exclusive OR function represents a data bit (such as D(1, P)) of arow of the corner turning memory.

An exemplary correlation triangle 2660 in reference to a threshold value2665 is illustrated in FIG. 26. Within the correlation threshold sense2645, a threshold detector provides a high signal when the accumulatedoutputs of the exclusive OR function exceed the threshold value 2665and, otherwise, provide a low signal. A resulting pulse 2670 asillustrated in FIG. 26 may be employed to ascertain (e.g., estimate) alocation of a peak of the correlation triangle 2660.

Looking at any of the sample(s) and in accordance with theaforementioned discussion regarding FIGS. 23, et seq. by performing abit-for-bit exclusive OR function between the reference bits R andderived sample bits T from the sample bits S and shifting the derivedsample bits T from the sample bits S one bit at a time and performinganother bit-for-bit exclusive OR, the correlator derives a correlationtriangle when the reply code from the RFID tag is present (which may besufficient to detect the RFID tag). If the correlation triangle isundistinguishable (maybe weak signal or high ambient noise), thecorrelation subsystem then repeats the aforementioned function (andmaybe many times over) on yet another reply code(s) and the correlatorderives yet another correlation triangle, in such circumstances, theresults are then integrated (e.g., bit for bit summation) of thecorrelation triangles to derive a non-coherent integration of multiplereply codes.

As illustrated and described, the previously introduced embodiments ofthe correlation subsystem employ a gated correlation operation as thesignal (reply code) to be correlated is captured in a block and thengated through the correlation subsystem in a serial fashion, at leastone bit at a time. For strong signal conditions, a single sampling andcorrelation will often be sufficient to determine a presence of an RFIDtag. For conditions where the signals are not as strong, the number ofsummed samples increases to be greater than one. An improvement in theSNR may result, in part, from a summing operation and the improvementmay increase as the square root of the number of samples averaged beforebeing input to the decision subsystem. The aforementioned summing orintegration functionality may be referred to as non-coherentintegration.

Turning now to FIGS. 27 and 28, illustrated are block diagrams ofembodiments of a Fast Fourier Transform (FFT) operation employable witha correlation subsystem associated with the control and processingsubsystem of the interrogator constructed according to the principles ofthe present invention. As mentioned above, convolution theorem may beperformed in conjunction with a FFT operation. A FFT is performed on thereply code and the stored reference code from the initialization stage(see FIG. 27). The reply code and reference code (or selected bits orsample bits thereof) are then complex multiplied and converted back tothe time domain by an Inverse Fast Fourier Transform (“IFFT”). Zeropadding is employed to substantially prevent aliasing.

With the exception of the FFT operations, the correlation subsystem isanalogous to the correlation subsystems described above. The convolutiontheorem and the use of FFTs provides a computationally efficienttechnique of implementing a correlation subsystem. Additionally,aliasing in regions of non-interest (i.e., where a correlation triangleis not likely to exist) should not harm the results so that the lengthof the FFT may be shortened to improve computational efficiency. Also,in those instances where the range of variation is small, it may bepreferable to employ dot product correlations for even furthercomputational efficiency.

Referring once again to FIG. 27, incoming data is collected as a blockof complex I and Q values 2710 followed by removing the DC terms 2715.These values are correlated 2720 against a reference code 2725 using theconvolution theorem. Subsequent to correlation, a square law envelopedetection function 2730 obtains amplitude information. This is theninput to the corner turning memory function 2735 followed by a summer2740 and then a threshold and detection function 2745, of which werediscussed above.

Referring once again to FIG. 28, the specifics of the correlationoperation 2720 is illustrated in detail. The incoming data of length S2805 is increased to length M by zero padding 2810 to assure the data isof an appropriate length for an FFT operation 2815. In the same manner,the normalized length R of the reference code 2830 is also increased tolength M by a zero pad 2835, to which is then performed a length M FFT2845 and a complex conjugate 2850 of the result performed. A complexterm by term multiplication 2820 is performed and a length M IFFT 2825is performed to complete the function.

In the presence of noise, or during very weak signal conditions, thecorrelation subsystem may erroneously correlate on noise. Theinterrogator substantially reduces those effects by comprehending thatactual correlations do not have a single correlation peak, but a majorpeak that is surrounded by smaller correlation peaks. The aforementionedcharacteristics are referred to as sidelobes or time sidelobes.

Turning now to FIG. 29, illustrated is a waveform diagram demonstratingthe sidelobes associated with the correlation subsystem in accordancewith the principles of the present invention. Understanding the natureof the sidelobes and using their characteristics within a predetectingfunction can enhance the correlation subsystem of the interrogator. Asillustrated, the correlation includes a major peak 2910 (referred to as“prompt”) and two smaller peaks (generally referred to as “early” 2915and “late” 2920) about the major peak. By averaging the noise in theearly and late regions and comparing those values to noise levelsrecorded when it was known that no signal was present, additionalconfirmation is obtained that, in fact, an RFID tag is responding evenif the RFID tag is not uniquely identifiable in a single response at thepresent signal levels. Then, by averaging multiple responses thatcorrespond to RFID tag responses, the SNR will be raised to a levelwherein substantially unambiguous detection occurs.

In this instance, the reply code of an RFID tag is not being detected,but the interrogator is detecting a change in ambient noise thatsubstantially increases the probability that an RFID tag is indeedpresent. For example, sampling in all three regions and having the noiselevel be the same is a good indication that an RFID tag is not presentand therefore that the sample should be discarded. However, sampling inall three areas and finding that the early and late levels are aboutequal and the middle level is larger is a good indication that aresponse from an RFID tag is in fact present and that this sample shouldbe added into the averaging function. Clearly discarding samples that donot pass the early/late noise test will certainly discard data of actualRFID tags. That is a small price to pay, however, for not undulycorrupting the average with samples that do not in fact contain a replycode from an RFID tag. Sampling for slightly longer times compensatesfor the reduction in samples used. The control and processing subsystemcan maintain a running total of how many samples were discarded so thatthe number of samples averaged will remain valid.

Regarding the timing of the responses from the RFID tag, a tracking ofthe reply codes may suggest that the reply code is early, prompt orlate. If the tracking suggests that the reply code is prompt (promptoutput greater than early and late output), then a gating function isproperly aligned to provide a significant correlator output. If thetracking suggests that the reply code is early, then the earlycorrelator output is significant as compared to the late correlatoroutput and the correlation subsystem is tracking too early and therequisite adjustment may be performed. An opposite adjustment may beperformed if the tracking suggests that the reply code is late.

Another approach is to use a tracking loop that uses past successfuldetection performance to establish a gating process for subsequentcorrelations. In yet another embodiment relating to the correlation ofthe reply codes from the RFID tags is to perform FFTs on both thereference code and a gated sample of the reply codes from the RFID tags.Then, a convolution operation in “Fourier Space” may be performedemploying the convolution theorem. The convolution theorem states thatthe convolution of two functions is the product of the Fouriertransforms thereof. An output of the correlation operation is typicallyenvelope detected and several outputs may be averaged in a summingoperation that preserves time characteristics of each individualdetection.

Turning now to FIG. 30, illustrated is a block diagram of an embodimentof a predetecting function operable with a correlation subsystemassociated with a control and processing subsystem of an interrogatorconstructed according to the principles of the present invention. Asmentioned above, multiple correlators in conjunction with thecorrelation subsystem within an interrogator may be employed toadvantage. The multiple correlators can be used to detect multiple RFIDtags simultaneously and, in that instance, the multiple correlatorstypically operate independently. Alternatively, multiple correlators canbe assigned to find a single RFID tag wherein each correlator can begiven a slightly different area of time or phase space to search. Thisis especially useful when timing of the response from the RFID tag to atransmit query is not known. Of course, the correlators may be realizedin software and/or hardware.

Incoming data is collected as a block of complex I and Q values 3010followed by removing the DC terms 3015. These values are correlated 3025against a reference code 3020 using the convolution theorem. Subsequentto correlation, a square law envelope detection function 3030 obtainsamplitude information. An alignment and predetection function 3035follows whose output is then input to the corner turning memory function3040 followed by the summer 3045 and then a threshold and detectionfunction 3050, of which were discussed above.

It is further within the broad scope of the present invention to employmultiple antennas in accordance with the interrogator and wherein themultiple antennas are employed for diversity and different correlatorsmay be assigned to specific antennas. Under such circumstances, postcorrelation and/or post averaging results may be combined to provideadded detection sensitivity. To perform the aforementioned functions,the interrogator may include multiple RFID sensing subsystems, each witha separate orthogonal input from an antenna(s).

Turning now to FIGS. 31 to 34, illustrated are waveform diagramsdemonstrating exemplary performances of an interrogator according to theprinciples of the present invention. More specifically, FIG. 31illustrates a correlator response with no averaging for an RFID tag atabout 29 feet. It is to be noted that a conventional reader would nottypically detect the RFID tag at this distance. These conditions areconsidered to be docile signal conditions as the SNR is quite large evenconsidering the RFID tag is 29 feet from the interrogator. FIG. 32illustrates the same conditions as above with the addition of employinga correlation operation with multiple correlations averaged together.Note that the effective noise has been reduced. With respect to FIG. 33,the effect of adding approximately 14 dB of noise to the data andwithout averaging is demonstrated. The signal is obscured. FIG. 34 showsthe results of applying averaging to the previously described conditionsfor the interrogator. Note that the distinct correlation signature hasbeen restored indicating that even under these extreme conditions, theRFID tag is detected with high reliability.

Exemplary embodiments of the present invention have been illustratedwith reference to specific electronic components. Those skilled in theart are aware, however, that components may be substituted (notnecessarily with components of the same type) to create desiredconditions or accomplish desired results. For instance, multiplecomponents may be substituted for a single component and vice-versa. Theprinciples of the present invention may be applied to a wide variety ofapplications to identify and detect RFID objects. For instance, in amedical environment, instrument kits including a plurality of RFIDobjects can be scanned in situ to log the contents thereof into aninterrogator and subsequently the instrument kit can be scanned by theinterrogator to verify the contents, the integrity of the contents(including expiration dates for time sensitive objects) and the like.The increased sensitivity of the interrogator according to theprinciples of the present invention opens up many new opportunities(e.g., supply chain management in consumer related retail applications,security applications, etc.) for the interrogation system disclosedherein.

For a better understanding of communication theory and radio frequencyidentification communication systems, see the following references “RFIDHandbook,” by Klaus Finkenzeller, published by John Wiley & Sons, Ltd.,2nd edition (2003), “Technical Report 860 MHz-930 MHz Class I RadioFrequency Identification Tag Radio Frequency & Logical CommunicationInterface Specification Candidate Recommendation,” Version 1.0.1,November 2002, promulgated by the Auto-ID Center, MassachusettsInstitute of Technology, 77 Massachusetts Avenue, Bldg 3-449, CambridgeMass. 02139-4307, “Introduction to Spread Spectrum Communications,” byRoger L. Peterson, et al., Prentice Hall Inc. (1995), “ModernCommunications and Spread Spectrum,” by George R. Cooper, et al.,McGraw-Hill Book Inc. (1986), “An Introduction to StatisticalCommunication Theory,” by John B. Thomas, published by John Wiley &Sons, Ltd. (1995), “Wireless Communications, Principles and Practice,”by Theodore S. Rappaport, published by Prentice Hall Inc. (1996), “TheComprehensive Guide to Wireless Technologies,” by Lawrence Harte, et al,published by APDG Publishing (1998), “Introduction to Wireless LocalLoop,” by William Webb, published by Artech Home Publishers (1998) and“The Mobile Communications Handbook,” by Jerry D. Gibson, published byCRC Press in cooperation with IEEE Press (1996). For a betterunderstanding of conventional readers, see the following readers,namely, a “MP9320 UHF Long-Range Reader” provided by SAMSysTechnologies, Inc. of Ontario, Canada, a “MR-1824 Sentinel-Prox MediumRange Reader” by Applied Wireless ID of Monsey, N.Y. (see also U.S. Pat.No. 5,594,384 entitled “Enhanced Peak Detector,” U.S. Pat. No. 6,377,176entitled “Metal Compensated Radio Frequency Identification Reader,” andU.S. Pat. No. 6,307,517 entitled “Metal Compensated Radio FrequencyIdentification Reader”), a “2100 UAP Reader,” provided by IntermecTechnologies Corporation of Everett, Washington and a “ALR-9780 Reader,”provided by Alien Technology Corporation of Morgan Hill, Calif. Theaforementioned references, and all references herein, are incorporatedherein by reference in their entirety.

Also, although the present invention and its advantages have beendescribed in detail, it should be understood that various changes,substitutions and alterations can be made herein without departing fromthe spirit and scope of the invention as defined by the appended claims.For example, many of the processes discussed above can be implemented indifferent methodologies and replaced by other processes, or acombination thereof, to form the devices providing reducedon-resistance, gate drive energy, and costs as described herein.

Moreover, the scope of the present application is not intended to belimited to the particular embodiments of the process, machine,manufacture, composition of matter, means, methods and steps describedin the specification. As one of ordinary skill in the art will readilyappreciate from the disclosure of the present invention, processes,machines, manufacture, compositions of matter, means, methods, or steps,presently existing or later to be developed, that perform substantiallythe same function or achieve substantially the same result as thecorresponding embodiments described herein may be utilized according tothe present invention. Accordingly, the appended claims are intended toinclude within their scope such processes, machines, manufacture,compositions of matter, means, methods, or steps.

What is claimed is:
 1. An interrogator comprising a control andprocessing system operable on a processor configured to: load areference code including at least a plurality of bits of a reply codefrom a radio frequency identification (RFID) tag associated with an RFIDobject; produce an excitation signal for said RFID tag; receive at leasta portion of said reply code from said RFID tag in response to saidexcitation signal; and compare said reference code to said at least saidportion of said reply code from said RFID tag to verify a presence ofsaid RFID tag.